Having a question about translating phpBB 3.1? Want to discuss and collaborate with people currently translating phpBB 3.1? Here would be the correct place to do so.
I have collected the diff files for each phpBB update into zip files - if you are trying to upgrade an old translation, you can go through these one by one.
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There is no direct big diff available fo what you need, you will have to work along one after the other, or create a diff from the GitHub project yourself.