updating locale files (by removing and adding a newer version of a xml file)
You should be aware that if you add locale files from a newer version of OJS to your not updated local installation, you may end up with unexpected errors. The locale files are part of the OJS releases, and you might see results like
- in the newer locale file we removed translations that your older installation still needs (because you only updated locale files, but not the actual code that uses the translations)
So I would always advise to update your OJS installation regularly and thus getting all the local updates.
local modifications to your installation
If you want to change the existing translation, e.g. because you want to see a different language in certain UI elements, you can modify the files. Please note that the next upgrade probably will overwrite your changes.
stuck with older OJS version
If you are stuck with an older OJS version and cannot upgrade, but want to profit from translation updates, you could identify missing keys (e.g. by using the translator plugin or by spotting untranslated text in the UI) and search the most recent XML files (from GitHub) for these keys to then add them locally.