Improve loadingScreen Spanish translation

[dbnicholson]

This was discussed in https://github.com/endlessm/kolibri-installer-android/issues/187 and some other places. Currently the loadingScreen Spanish translation isn't used on many Spanish locales. The problem is that the translation loading is done only for an exact locale in navigator.languages without any of the heuristics that Kolibri's library provides. That means that unless navigator.languages includes es-419, you won't get Spanish translations. As was found in https://github.com/endlessm/kolibri-installer-android/issues/187, there are many Android webview cases where this isn't the case.

I think the solution here is simple:

• If matching a translation from an exact locale fails, try again with just the language (i.e., the region stripped).
• Change our translations to be regionless. I.e. rename es-419 to just es. Maybe in the future we'd actually have regional translations, but I highly doubt it. Certainly today there's only a single Spanish translation. For completeness I'd also rename en-US to en.

[erikos]

https://github.com/endlessm/kolibri-explore-plugin/pull/894 has been merged. Waiting for a plugin release.

[erikos]

Build 7.10.427 works fine. QA pass.