Add multiple languages

[HugoFara]

Currently the app (and documentation) is available only in English.

Many language learners are not fluent in English and may not comfortable with this single language.

For secondary languages Mandarin and Spanish are good targets.

Also, that could integrate nicely with the "learner's primary language" when creating a new language. The language of the app would be its primary language.

[HugoFara]

I have added machine-translated strings for several languages: Spanish, Mandarin Chinese (Simplified), French, German, Brazilian Portuguese and Japanese. Will probably add more in the near future (I am currently rate-limited).

Translations are in the locale. Any human review is welcome (and needed)!

The documentation is not translated yet, it is a good next candidate.