I’ve finished the Spanish locale translation. Here are my options and reasons. Please discuss and propose.

Sources:
The ‘Real Academia Española de la Lengua’ is the official Academy. It’s not only for Spain, but it’s in contact with the net of Academies of the Spanish speaking countries.
http://buscon.rae.es/dpdI/SrvltConsulta?lema=abreviatura
http://buscon.rae.es/dpdI/apendices/apendice2.html
Ortografía de la Lengua Española, 2010.

EDIT: General observation
The abbreviations with several elements must have a no-break space between them (Unicode A0; more correct would be the narrow no-break space, Unicode 202F; cf. Ortografía de la Lengua Española, V.3.2.5.3)

Anonymus
The abbreviations retain the tilde, from ‘anónimo’ comes ‘anón.’
<term name="anonymous" form="short">anón.</term>

No date
The RAE (Real Academia Española de la Lengua) offers several abbreviations for ‘no date’: ‘s. f.’ ‘s/f’ ‘s. d.’ It seems that the more appropriate in a citations and bibliography context is ‘s. d.’ (from Latin, ‘sine data’). But in a couple of forums it wasn’t understood (http://forums.zotero.org/discussion/10859) (http://blog.apastyle.org/apastyle/2010/01/the-generic-reference-when.html#tpe-action-posted-6a01157041f4e3970b0120a7d6fff6970b). I prefer the ‘s. f.’ option; that’s more understandable.
<term name="no date" form="short">s. f.</term>

Presented at
‘presented at’ has an article in the locale files I’ve checked, but I’m not sure if the following word will be masculine or feminine. I’ve put a colon.
<term name="presented at">presentado en:</term>

Ordinals
I’ve seen that ‘ordinal’ is only used with ‘edition’ in so many CSL, that is feminine in Spanish. Perhaps it’s better to put the feminine ordinals and risk less errors (is there a CSL that uses it with masculine?).
The formal form has a period: ‘1.ª’ ‘3.ª’ It can’t be omitted (cf. Ortografía de la Lengua Española, V.3.2.2)
<term name="ordinal-01">.ª</term> (etc.)
<term name="long-ordinal-01">primera</term> (etc.)

Sub verbo
Is Spanish we use ‘sub voce’, it has the same meaning. The correct plural, I presume, is ‘sub vocibus’, but I haven’t seen it.
<term name="sub verbo">
<single>sub voce</single>
<multiple>sub vocibus</multiple>

Issue
The official abbreviation for ‘número’ is ‘n.º’, with the period. I know we usually write nº, but that isn’t formal (Cf. Ortografía de la Lengua Española, V.3.2.2).
<term name="issue" form="short">n.º</term>

Page
The abbreviations for ‘page’ (página) may be ‘p.’ ‘pg.’ ‘pág.’. The three options are correct. In citations the optimized option is ‘p.’ (and ‘pp.’ for the plural). Some Spanish Handbooks of Style require it. (http://forums.zotero.org/discussion/3561/multiple-citation-with-same-author/#Item_7)
<term name="page" form="short">
<single>p.</single>
<multiple>pp.</multiple>
</term>

Paragraph and section symbols
In Spanish § is the symbol for ‘paragraph’, and is the same for ‘section’. The symbol ¶ means «información complementaria (Filol.)» (http://buscon.rae.es/dpdI/apendices/apendice4.html)
Moreover, the symbols don’t change for the plural (Cf. Ortografía de la Lengua Española, 2010, III.4.9; V.3.2.4a).
<term name="paragraph" form="symbol">
<single>§</single>
<multiple>§</multiple>
</term>
<term name="section" form="symbol">
<single>§</single>
<multiple>§</multiple>
</term>