I'm guessing a bit because I don't have any translations installed, BUT you should find a/some .po (or maybe .pot) files in your installation that define the translations.
However, these are part of GIMP, and are controlled the same way as all the rest of the code, so if you were to make local changes, they would be over-written next time you updated GIMP.
I don't know how it works for separate scripts/plug-ins, but you may be able to contribute to the translations in general. See the GIMP developers page for a start:
https://www.gimp.org/develop/Try asking the question on the developers mailing list.