Hi Lukasz,
The approach we used is not ideal and it is just a workaround.
I agree that the problem that the initial barrier becomes higher than using transifex.
I would like to share our experience that Transifex is not ideal for large documents.
I hope this is a good input for requirements when considering another translation system
or improving Transifex.
We first use Transifex to translate strings, and after completing the first round
of translations, we use GitHub. It lowered the initial barrier.
During the review phase, only a few folks including me edited PO files directly
and other folks focused on the review (read the drafts and comments on GitHub).
It can be done through the web interface. There were few confusions and it worked.
However, this process requires some person(s) who edit/update the PO files
and it may increase the barrier to join translations.
Thanks,
Akihiro