To add onto @luceos's response since he so happened to post his reply right as I was typing my own... 😂🤗
If you take a look at the locale folder in your fork, you can see that there are already translations for 3rd party extensions such as
flagrow-image-upload.yml (which are deprecated btw).
flagrow-image-upload for example.
You'd first create a
.yml file for the extension you'd want to create a translation for (for your language pack) in your
locale directory with a unique name. The trend in this community seems to be vendor and then extension name such as the example given here.
As @luceos also mentioned, the translation keys need to be on par with the extension's translation keys as shown here for this specific example.
Most importantly, as @luceos also mentioned again, you will need to return the LanguagePack extender as shown here as well.
If you need help or have any further inquiries, we are here to support you along your way ❤️