This is a repository to allow the community to contribute fan translations. Each contribution is automatically packaged up as a new release, so that language contributors no longer have to wait long times for their contributions to get included in the next release.
The same way you install the BTW mod: you put it in your mods folder alongside the BTW jar.
There were issues with quality control for languages not spoken by the CE team, as well as liability issues, and it sometimes took months for translations to make it into a mod release. We realize it's annoying that your hard work is not available by default, but this truly is the best option for all parties involved.
The best way is for you to make what's called a PR (Pull Request) here on Github. Someone will then include it for you, which will instantly trigger a new release. Alternatively if you find Github too confusing, you can try to find someone on the Discord who will submit it for you.