Enable German l10n for FirefoxOS

According to bug #43 not every language is translated yet.

It seems, that the German l10n is there, but not enabled, yet.
This pull request adds the missing parts to manifest.webapp.

I couldn't check it on my computer due to technical difficulties with Firefox :-(
However, all strings seem to be translated, so I'm hopefully.

I couldn't find a CONTRIBUTION file, so I don't know, whether tests etc. are required.