I had to patch i18n_taxonomy.module to make autocompletion work for me. I'll upload the patch right after this post.
Here's the problem and some words on the resolution:
I have configured a textfield to autocomplete on a 'per language terms' vocabulary. Once I start typing I got a JS error, because the AJAX callback returns an empty string.
The AJAX call looks like http://host//taxonomy/autocomplete//
In i18n_taxonomy.module the path "/taxonomy/autocomplete" is routet to 'i18n_taxonomy_autocomplete' by hook_menu_alter.
Here's where the problesm start: i18n_taxonomy_autocomplete() expects a vocabulary Object, a language code and the tags as input. But from the request we get a fieldname, some tags and no langcode.
My solution is to reroute '/taxonomy/autocomplete' to a tiny function which figures out the respective vocabulary and determines the language, and pass this info to i18n_taxonomy_autocomplete()
Determination of the language is easy in my case, because I can rely on i18n_language(). However I can imagine cases where the language depends on the context. (e.g. editing a french node in a german UI). So my fix is limited to some special cases.
Hope it helps someone out there anyway...