> Is it possible to commit the translation also to the RELEASE_0_91_BRANCH
Yes, but some strings have changed and using the trunk po file *could* break strings that are correctly translated in 0.91.
One thing you can do is update the trunk po file with the 0.91.x branch inkscape.pot template and see how many strings (and which ones) are marked fuzzy or untranslated compared to the 0.91.x po file.
> Is it possible to commit the translation also to the RELEASE_0_91_BRANCH
Yes, but some strings have changed and using the trunk po file *could* break strings that are correctly translated in 0.91.
One thing you can do is update the trunk po file with the 0.91.x branch inkscape.pot template and see how many strings (and which ones) are marked fuzzy or untranslated compared to the 0.91.x po file.