Support for special testing "xx" language

[lslezak]

Problem

• It is difficult to find out which texts are marked for translation
• Or to find out untranslated string

Solution

• "Translate" all texts in the _() and n_() functions, just convert them to X sequences
  • Just be careful of special %s or %d sequences, those should be kept

Notes

• This of course handles only the texts generated by the web UI itself, it cannot mark the texts coming from the backed

Screenshots

• Use ?lang=xx URL parameter, this can be entered only manually, we should never allow this in the UI

[imobachgs]

Interesting idea. LGTM. 😃

[coveralls]

coverage: 72.05% (-0.09%) from 72.14% when pulling ad6ca30 on xx_lang into fff5ddf on master.