The following code segment in my style file correctly changes the term 'accessed', but does nasty things to some (not all!) of the other terms like 'and', 'edition': they get truncated to the first letter ("u" instead of "und" for 'and'; "A" instead of "Aufl." for 'edition' ...)

<terms>
<locale xml:lang="de">
<term name="accessed">zugegriffen am</term>
</locale>
</terms>

Also, is it possible to add a missing term?

I'm in need of a term I call the 'here' term. It is used when you refer to a specific page (or page range) of an article and the reference is the full bibliographic entry (some styles for humanities handle it this way for the first occurrence):

Wandhoff, Haiko: “Auf dem Weg zu einer Universalgeschichte des Textes,” Zeitschrift für Germanistik 3 (1997), S. 599-606, hier S. 601.

The "hier" is needed to clarify that *here* you point to page 601, whereas the article as a whole runs from page 599 to 606 in the journal.

For my styles I have (for now) found no other solution than to "hard code" the missing "hier" literally into the corresponding macros of the style files.