You need to find zotero.jar, use an archive manager to replace the .dtd (or add a new one) under /chrome/locale and the respective subfolder, and repackage.
It's not something that's expected of people doing translations, obviously.
@samkultz : what adamsmith said. But if you need to know the context of a string, search for its name (aka "key" on Transifex) in the source (chrome/content/zotero/) – that can help.
Thank you guys for your help. I just wanted to see how the modified translations would look like in the day-to-day zotero use, and it's working, soon I'll post them on the translators website.
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It's not something that's expected of people doing translations, obviously.