[TASK] L10n: Pull translations from Transifex

Update translations for already existing languages
[TASK] L10n: Re-order languages.json, code first

By resorting the JSON languages array, code field first, maintaining
the file gets easier, as folders are sorted the same way.
[TASK] L10n: Use ISO 639-1 codes

Rename language files to match ISO 639-1 language codes, to comply
with Transifex naming.
[TASK] L10n: Update German, Russian, Turkish

Import new translations from Transifex.
[TASK] L10n: Support ISO 639-1 codes

In the hardcoded language links (footer) and during pageload, use and
support the ISO 639-1 codes in both short and long form
[FIX] JS debug mode: Partially revert 21a15d1

In 21a15d1, JavaScript debug mode accidentally got always disabled.
Fix this bug by re-enabling debug mode on debug compiles.
Merge pull request #1619 from jublonet/feature/update-po

[TASK] Use ISO 639-1 codes, pull translations