Add fallback method when target_language is empty

It seems that Transifex doesn't assign any value to the target-language
property in the file element (XLIFF). This commit updates i18n:update-fixtures
so the language code is extracted from the file path when it was not provided
in the XLIFF.
Latest translations (just fixtures this time)

The previous commit fixed the merge of translations pulled from Transifex
and this is the proof that it's working! Props to david for his great work
building the i18n tasks that allowed to automate this process so easily!