fix #30266: Some button labels are not translated

Fixed by replacing QDialogBottonBox by QPlatformTheme, which seems what
Qt 5.2 (?) and 5.3 (!) needs as the context, to load the right
translations for these button labels.

Also some more cosmetic fixes for the German translation
Merge pull request #1272 from Jojo-Schmitz/translation

fix #30266: Some button labels are not translated