Small typo stops translation cache from actually working.

[beniwohli]

In multilingual.translation.fill_translation_cache, there's a check if the attribute _trans_code_XX exists (XX being the language code). But this attribute is actually called _trans_language_code_XX, so the test is always false and the translation cache isn't used, resulting in additional queries when translated fields are accessed.

I commited a fix to this issue on my fork: piquadrat/django-multilingual-ng@8a717010b08248aff67d . I wasn't able to create a pull request with this single commit, but cherry-picking should work.