Currently Plupload is translated into 30+ languages. Since in the Core API there is nothing to translate really, except the error messages, translations on big part cover the widgets (UI and Queue).
We bundle language packs with the Plupload packages - they are available under
js/i18n/ folder. Alternatively you can always download them from the GitHub.
To activate the language pack you simply need to include the corresponding js file after the Plupload scripts, e.g.:
In case you are not satisfied with the quality of translation, or if there's no translation for your language at all, you can contribute to it yourself. We've moved our language management facilities to Transifex. After you have a Transifex account you can join a translation team for your language or request a new one. All contributions will be automatically included in the next official release.
We used to accept pull requests with translations in the past, but we've deprecated this approach now. Use Transifex instead.