Added a test suite for languages

Hi there!

So I wrote a small test suite to ensure every string is translated in every language. You run it with `npm test`. The tests are not passing for now because the subtitles language strings are not translated in every language:

```
Property french not found in es.json
Property english not found in es.json
Property spanish not found in es.json
Property swedish not found in es.json
Property portuguese not found in es.json
Property romanian not found in es.json
Property turkish not found in es.json
Property swedish not found in fr.json
Property french not found in ja.json
Property english not found in ja.json
Property spanish not found in ja.json
Property swedish not found in ja.json
Property portuguese not found in ja.json
Property romanian not found in ja.json
Property turkish not found in ja.json
Property french not found in nl.json
Property english not found in nl.json
Property spanish not found in nl.json
Property swedish not found in nl.json
Property portuguese not found in nl.json
Property romanian not found in nl.json
Property turkish not found in nl.json
Property french not found in pt-br.json
Property english not found in pt-br.json
Property spanish not found in pt-br.json
Property swedish not found in pt-br.json
Property portuguese not found in pt-br.json
Property romanian not found in pt-br.json
Property turkish not found in pt-br.json
Property french not found in pt.json
Property english not found in pt.json
Property spanish not found in pt.json
Property swedish not found in pt.json
Property portuguese not found in pt.json
Property romanian not found in pt.json
Property turkish not found in pt.json
Property french not found in ro.json
Property english not found in ro.json
Property spanish not found in ro.json
Property swedish not found in ro.json
Property portuguese not found in ro.json
Property romanian not found in ro.json
Property turkish not found in ro.json
Property swedish not found in sv.json
Property french not found in tr.json
Property english not found in tr.json
Property spanish not found in tr.json
Property swedish not found in tr.json
Property portuguese not found in tr.json
Property romanian not found in tr.json
Property turkish not found in tr.json
```

But put aside from this PR, maybe this isn't right: why translate a language name in each other languages? First, this is replication and leads to human errors (for example `de.json` has every language name translated BUT Deutsch...)
Why not let "English", "Français", and so on for each language? After all, **if I can't read the subtitles I don't expect the subtitles button to have a translated title**.

Anyway, let me know what you think of the test cases, is it useful, and what about those last thoughts ^^