Importing old translations
1. Install poedit
2. get an updated .pot file
3. download old per-language .po file from https://github.com/UnvanquishedAssets/unvanquished_src.dpkdir/tree/master/translation/game
4. In poedit, Edit → Preferences → TM → Manage; import existing .po
5. In poedit, import .pot file, turn it into a per-language .po file
6. In poedit, Catalog → Pre-Translate, check "Only fill in exact matches", you may also want to check the other one to not mark auto-translated strings as needing review (as you prefer).