Merging .po-files with Git is painful.
of making Git more clever when trying to merge .po-files. I believe however that this is not enough as it can still produce invalid .pos (usually due to double definitions), which can seriously confuse gettext afterwards.
Lacking any more secure solution, I think you should avoid editing po files in different branches at the same time. In order to not accidentally produce invalid merges I additionally disable merging for po files altogether, by adding the following
.gitattributes file to the root of the repository:
*.po merge=binary *.pot merge=binary