I used the reconvert_l10n.py file to convert the po file to json.
But i don't like that it modifies the po file also.
In my case it incorrectly converted all the character "æøåÆØÅé" due to incorrect encoding so i had to fix them manually afterwards.
https://github.com/teeworlds/teeworlds/blob/master/scripts/reconvert_l10n.py#L50-L57
Changing:
po = polib.pofile(open(filename).read())
To:
po = polib.pofile(open(filename, encoding='utf-8').read())
Would fix the proble...