Just solved the language-problem installing Quick Locale Switcher !
But still I could use some help concerning the citation style.

Hi adamsmith!
Thank you for the links!
I already managed to change some issues in order to get the citation as I described. There is just one little problem that I don't seem to solve:
The bibliography style that I would like to use is like this:
Young, Crawford (1976), The Politics of Cultural Pluralism, Madison: University of Wisconsin Press.
The only thing that I didn't find out was how to put a colon in between the city of publication and the publishing house. I have no idea where I find the suitable position to change that. Could you please help me?? This is the encoding I made:

<?xml version="1.0" encoding="UTF-8"?>
<style xmlns="http://purl.org/net/xbiblio/csl" xml:lang="en" class="in-text" >
<info>
<title>Schumpeter(dev)</title>
<id>http://www.zotero.org/styles/agu</id>;
<link href="http://www.zotero.org/styles/agu"/>
<author>
<name>Pia Bergmann</name>
</author>
<category term="author-date"/>
<category term="politics"/>
<updated>2011-02-15</updated>
<summary>Style for the Schumpeter Research Project of the German Institute of Global and Area Studies</summary>
<link href="http://www.giga-hamburg.de/"/>
</info>
<macro name="editor">
<names variable="editor" delimiter=", ">
<label form="verb" text-case="lowercase" suffix=" "/>
<name and="text" initialize-with=". " delimiter=", " name-as-sort-order="all"/>
</names>
</macro>
<macro name="anon">
<text term="anonymous" form="short" text-case="capitalize-first"/>
</macro>
<macro name="author">
<names variable="author">
<name name-as-sort-order="first" and="text" sort-separator=", "
delimiter=", " form="long"
delimiter-precedes-last="always" initialize-with=". "/>
<label form="short" prefix=" (" suffix=".)" text-case="capitalize-first"/>
<substitute>
<names variable="editor"/>
<text macro="anon"/>
</substitute>
</names>
</macro>
<macro name="author-short">
<names variable="author">
<name form="short" and="text" delimiter=", " initialize-with=". "/>
<substitute>
<names variable="editor"/>
<names variable="translator"/>
<text macro="anon"/>
</substitute>
</names>
</macro>
<macro name="access">
<choose>
<if variable="URL">
<text term="online" prefix="[" suffix="]"/>
<text value=" Available from: " />
<text variable="URL"/>
<group prefix=" (" delimiter=" " suffix=")">
<text term="accessed" text-case="capitalize-first"/>
<date variable="accessed" >
<date-part name="day" suffix=" "/>
<date-part name="month" suffix=" "/>
<date-part name="year"/>
</date>
</group>
</if>
</choose>
</macro>
<macro name="title">
<choose>
<if type="book">
<text variable="title" font-style="italic"/>
</if>
<else>
<text variable="title"/>
</else>
</choose>
</macro>
<macro name="publisher">
<group delimiter=", ">
<text variable="publisher"/>
<text variable="publisher-place"/>
</group>
</macro>
<macro name="year-date">
<choose>
<if variable="issued">
<date variable="issued">
<date-part name="year"/>
</date>
</if>
<else>
<text term="no date"/>
</else>
</choose>
</macro>
<macro name="published-date">
<choose>
<if type="article-newspaper">
<date variable="issued">
<date-part name="day" form="ordinal" suffix=" "/>
<date-part name="month" form="long"/>
</date>
</if>
</choose>
</macro>
<macro name="pages">
<text variable="page"/>
</macro>
<macro name="refpages">
<label variable="page" form="short" suffix=". "/>
<text variable="page"/>
</macro>
<macro name="edition">
<choose>
<if is-numeric="edition">
<group delimiter=" ">
<number variable="edition" form="ordinal"/>
<text term="edition" form="short" suffix="."/>
</group>
</if>
<else>
<text variable="edition" suffix="."/>
</else>
</choose>
</macro>
<macro name="doi">
<text variable="DOI" prefix="doi:"/>
</macro>
<macro name="container">
<group class="container" delimiter=", ">
<group delimiter=" ">
<text term="in" text-case="lowercase"/>
<text variable="container-title" font-style="italic"/>
</group>
<group delimiter=" ">
<text term="volume" form="short" suffix="."/>
<text variable="volume" />
</group>
<text macro="editor"/>
</group>
</macro>
<citation>
<option name="et-al-min" value="3"/>
<option name="et-al-use-first" value="1"/>
<option name="et-al-subsequent-min" value="3"/>
<option name="et-al-subsequent-use-first" value="1"/>
<option name="disambiguate-add-year-suffix" value="true"/>
<option name="disambiguate-add-names" value="false"/>
<option name="disambiguate-add-givenname" value="false"/>
<option name="collapse" value="year"/>
<sort>
<key macro="author"/>
<key macro="year-date"/>
</sort>
<layout prefix="(" suffix=")" delimiter="; ">
<group delimiter=": ">
<group delimiter=" ">
<text macro="author-short"/>
<text macro="year-date"/>
</group>
<text variable="locator"/>
</group>
</layout>
</citation>
<bibliography>
<option name="hanging-indent" value="true"/>
<option name="et-al-min" value="11"/>
<option name="et-al-use-first" value="1"/>
<sort>
<key macro="author"/>
<key macro="year-date"/>
<key variable="title"/>
</sort>
<layout>
<group delimiter=" " suffix=",">
<text macro="author" />
<text macro="year-date" prefix="(" suffix=")"/>
</group>
<choose>
<if type="book">
<group delimiter=", " prefix=" " suffix=".">
<text macro="title"/>
<group delimiter=" ">
<text variable="collection-title"/>
<text variable="collection-number"/>
</group>
<text macro="edition"/>
<text macro="editor" />
<text variable="genre"/>
<text macro="publisher"/>
</group>
</if>
<else-if type="chapter">
<group delimiter=", " prefix=" " suffix=".">
<text macro="title"/>
<text macro="container"/>
<text macro="refpages"/>
<text macro="publisher"/>
</group>
</else-if>
<else-if type="thesis">
<group delimiter=", " prefix=" " suffix=".">
<text macro="title"/>
<text variable="genre"/>
<text variable="page" suffix=" pp."/>
<text macro="publisher"/>
<date variable="issued">
<date-part name="day" suffix=" "/>
<date-part name="month" form="long"/>
</date>
</group>
</else-if>
<else>
<group delimiter=", " suffix="," prefix=" ">
<text macro="title"/>
<text macro="editor"/>
</group>
<group class="container" prefix=" " suffix=".">
<text variable="container-title" font-style="italic" form="short"/>
<group prefix=", " delimiter=", ">
<group>
<text variable="volume"/>
<text variable="issue" prefix=", "/>
<text macro="published-date"/>
</group>
<text macro="pages"/>
<text macro="doi"/>
</group>
</group>
</else>
</choose>
<text prefix=" " macro="access" />
</layout>
</bibliography>
</style>

I found this passage and now it works exactly the way I wanted. Thanks a lot!

<?xml version="1.0" encoding="UTF-8"?>
<style xmlns="http://purl.org/net/xbiblio/csl" xml:lang="en" class="in-text" >
<info>
<title>Schumpeter(dev)</title>
<id>http://www.zotero.org/styles/agu</id>;
<link href="http://www.zotero.org/styles/agu"/>
<author>
<name>Pia Bergmann</name>
</author>
<category term="author-date"/>
<category term="politics"/>
<updated>2011-02-15</updated>
<summary>Style for the Schumpeter Research Project of the German Institute of Global and Area Studies</summary>
<link href="http://www.giga-hamburg.de/"/>
</info>
<macro name="editor">
<names variable="editor" delimiter=", ">
<name name-as-sort-order="first" and="text" sort-separator=", "
delimiter=", " form="long"/>
<label form="short" prefix=" (" suffix=".)"/>
</names>
</macro>
<macro name="anon">
<text term="anonymous" form="short" text-case="capitalize-first"/>
</macro>
<macro name="author">
<names variable="author">
<name name-as-sort-order="first" and="text" sort-separator=", "
delimiter=", " form="long"/>
<label form="long" text-case="capitalize-first"/>
<substitute>
<text macro="editor"/>
<text macro="anon"/>
</substitute>
</names>
</macro>
<macro name="author-short">
<names variable="author">
<name form="short" and="text" delimiter=", " initialize-with=". "/>
<substitute>
<names variable="editor"/>
<names variable="translator"/>
<text macro="anon"/>
</substitute>
</names>
</macro>
<macro name="access">
<choose>
<if variable="URL">
<text value="at:" suffix=" "/>
<text variable="URL"/>
<group prefix=" (" suffix=").">
<date variable="accessed">
<date-part name="day" suffix=" "/>
<date-part name="month" suffix=" "/>
<date-part name="year"/>
</date>
</group>
</if>
</choose>
</macro>
<macro name="title">
<choose>
<if type="book">
<text variable="title" font-style="italic"/>
</if>
<else-if type="URL">
<text variable="title" font-style="italic"/>
</else-if>
<else>
<text variable="title"/>
</else>
</choose>
</macro>
<macro name="publisher">
<group delimiter=": ">
<text variable="publisher-place"/>
<text variable="publisher"/>
</group>
</macro>
<macro name="year-date">
<choose>
<if variable="issued">
<date variable="issued">
<date-part name="year"/>
</date>
</if>
<else>
<text term="no date"/>
</else>
</choose>
</macro>
<macro name="published-date">
<choose>
<if type="article-newspaper">
<date variable="issued">
<date-part name="day" form="ordinal" suffix=" "/>
<date-part name="month" form="long"/>
</date>
</if>
</choose>
</macro>
<macro name="edition">
<choose>
<if is-numeric="edition">
<group delimiter=" ">
<number variable="edition" form="ordinal"/>
<text term="edition" form="short" suffix="."/>
</group>
</if>
<else>
<text variable="edition" suffix="."/>
</else>
</choose>
</macro>
<macro name="pages">
<text variable="page"/>
</macro>
<macro name="refpages">
<text variable="page"/>
</macro>
<macro name="doi">
<text variable="DOI" prefix="doi:"/>
</macro>
<macro name="container">
<group class="container" delimiter=", ">
<group delimiter=" ">
<text macro="editor" prefix="in: "/>
<text term="" text-case="lowercase"/>
<text variable="container-title" font-style="italic"/>
</group>
<group delimiter=" ">
<text term="volume" form="short" suffix="."/>
<text variable="volume"/>
</group>
</group>
</macro>
<citation>
<option name="et-al-min" value="3"/>
<option name="et-al-use-first" value="1"/>
<option name="et-al-subsequent-min" value="3"/>
<option name="et-al-subsequent-use-first" value="1"/>
<option name="disambiguate-add-year-suffix" value="true"/>
<option name="disambiguate-add-names" value="false"/>
<option name="disambiguate-add-givenname" value="false"/>
<option name="collapse" value="year"/>
<sort>
<key macro="author"/>
<key macro="year-date"/>
</sort>
<layout prefix="(" suffix=")" delimiter="; ">
<group delimiter=": ">
<group delimiter=" ">
<text macro="author-short"/>
<text macro="year-date"/>
</group>
<text variable="locator"/>
</group>
</layout>
</citation>
<bibliography>
<option name="hanging-indent" value="true"/>
<option name="et-al-min" value="11"/>
<option name="et-al-use-first" value="1"/>
<sort>
<key macro="author"/>
<key macro="year-date"/>
<key variable="title"/>
</sort>
<layout>
<group delimiter=" " suffix=",">
<text macro="author" />
<text macro="year-date" prefix="(" suffix=")"/>
</group>
<choose>
<if type="book">
<group delimiter=", " prefix=" " suffix=".">
<text macro="title"/>
<group delimiter=" ">
<text variable="collection-title"/>
<text variable="collection-number"/>
</group>
<text macro="edition"/>
<text macro="editor" />
<text variable="genre"/>
<text macro="publisher"/>
</group>
</if>
<else-if type="chapter">
<group delimiter=", " prefix=" " suffix=".">
<text macro="title"/>
<text macro="container"/>
<text macro="publisher"/>
<text macro="refpages"/>
</group>
</else-if>
<else-if type="thesis">
<group delimiter=", " prefix=" " suffix=".">
<text macro="title"/>
<text variable="genre"/>
<text variable="page" suffix=" "/>
<text macro="publisher"/>
<date variable="issued">
<date-part name="day" suffix=" "/>
<date-part name="month" form="long"/>
</date>
<else-if type="article-journal">
<group delimiter=" " prefix=" " suffix=".">
<text macro="title" suffix=", in:"/>
<text macro="container"/>
<text macro="refpages"/>
</group>
</else-if>
</group>
</else-if>
<else>
<group delimiter=", " suffix="," prefix=" ">
<text macro="title"/>
<text macro="editor"/>
</group>
<group class="container" prefix=" " suffix=".">
<text variable="container-title" font-style="italic" form="short"/>
<group prefix=", " delimiter=", ">
<group>
<text variable="volume"/>
<text variable="issue" prefix=", "/>
<text macro="published-date"/>
</group>
<text macro="pages"/>
<text macro="doi"/>
</group>
</group>
</else>
</choose>
<text prefix=" " macro="access" />
</layout>
</bibliography>
</style>

Thank you fbennett for your fast help!
The italic issue is perfectly solved now!
But the punctuation did indeed break elsewhere.
Now every quotaion looked like this (with two commas after the year):

Cirincione, Joseph, Jon B. Wolfsthal, and Miriam Rajkumar (2002),, Deadly Arsenals: Tracking Weapons of Mass Destruction, Washington, D.C.: The Brookings Institution Press.

I fixed that but now the comma disappears in the report:

Flemes, Daniel (2006), Brazil’s Nuclear Policy - From Technological Dependence to Civil Nuclear Power, GIGA Working Paper, 23, Hamburg: GIGA German Institute of Global and Area Studies at: www.giga-hamburg.de/workingpapers.

Still the comma before the "at" is missing...but at least there is a point at the end!
I'll post the newest version and it would help me a lot if you could have another look at it!

This may be a little closer. Same address: https://gist.github.com/913322 I've restored the comma on the group delimiter, and dropped it instead from the data block. While it makes the style a little more difficult to interpret, putting punctuation on delimiters is better than putting it on affixes (especially the suffix), because it avoids duplicate punctuation.

I've also restored the change to the ID of the style again. There is a reason for that: the AGU style is a standard style from the Zotero repository, and if the ID of this style is not changed, it will be overwritten when you refresh styles in Zotero.

Finally, I've moved the terminal point suffix up to the "layout" tag, and added a delimiter="; " there. The layout delimiter provides punctuation between full citations, when citing in multiple entry mode. You might want to change the semicolon to a point, depending on how you want multiple cites to look.

Glad to hear it's getting closer; let me know if any further tuning is needed. (When posting code, it's good to use http://gist.github/com/ as a drop-box. It's hard to read code posted directly to the forum, and some things can break due to character conversion.)

also, these lines don't make sense and are not valid csl:
<else-if type="URL">
<text variable="title" font-style="italic"/>
</else-if>

there is no type URL - there is a variable URL or a type "webpage" - I guess you want the latter. You can either just delete this, or, probably better, change it to webpage and then simplify this in the bib section
<choose>
<if type="webpage">
<text macro="title" font-style="italic"/>
</if>
<else>
<text macro="title"/>
</else>
</choose>
to this

<text macro="title"/>

I found another error that I don't know how to deal with...
If the item type is a thesis, it shows really weird in the bibliography:

Kim, Hyung Min (2007), Social Network Conceptualizations of International System Structure and National Power:A Social Network Perspective on International Relations, Chapel Hill: University of North Carolina, Social Network Conceptualizations of International System Structure and National Power:A Social Network Perspective on International Relations.

I don't know why it is repeating the title, but of course, it shouldn't.
Adamsmith or fbennett could one of you be so kind and fix this for me??

I want it to be shown like this:

Last name, first name (year), Type, Title (italic), Place: University.

I tried to put the style into https://gist.github.com/43517ed1147def4475db.
I hope you can open it just like that (I didn't know whether a registration was needed or not)...
I'll post the style here anyway.
Thanks a lot in anticipation!

for gist.github - the link doesn't work - just past the text into the box and click create public gist, then copy the address in the URL bar - it should just have a number after the slash.
The problem is here - you're not properly closing the else-if on the thesis item type.
This:
<else-if type="thesis">
<group delimiter=", " prefix=" " suffix=".">
<text variable="genre"/>
<text macro="title" font-style="italic"/>
<text macro="publisher"/>
<date variable="issued">
<date-part name="day" suffix=" "/>
<date-part name="month" form="long"/>
</date>
<else-if type="article-journal">
<group delimiter=" " prefix=" ">
<text macro="title" suffix=", in:"/>
<text macro="container"/>
<text macro="refpages"/>
</group>
</else-if>
</group>
</else-if>
Should be changed to this
<else-if type="thesis">
<group delimiter=", " prefix=" " suffix=".">
<text variable="genre"/>
<text macro="title" font-style="italic"/>
<text macro="publisher"/>
<date variable="issued">
<date-part name="day" suffix=" "/>
<date-part name="month" form="long"/>
</date>
</group>
</else-if>
<else-if type="article-journal">
<group delimiter=" " prefix=" ">
<text macro="title" suffix=", in:"/>
<text macro="container"/>
<text macro="refpages"/>
</group>
</else-if>


I've got another question: I wanted the title of a conference paper to be shown in italic, but I don't know how is the type called, 'cause "conference paper" or just "paper" doesn't work...