How to use a translated taxonomy in autocomplete term-reference fields?
Are you using 'Entity Translation' or the core 'Content translation'?

I think the sub-module Multilingual select (from Internationalization) should handle this for 'Content translation' (didn't test it, though).

But for Entity Translation, this doesn't seem to work (yet): see Entity translation and the Multilingual select i18n submodule interactions and Language aware edit form and autocomplete.