You do not need to download the entire repository, you can download one language pack at this link.
If you want to translate the core, please use Crowdin. Third-party scripts (includes folder) or readme-*.html you need to translate manually and create a Pull Request.
Crowdin invitation link. Members with no contributions will be removed.
Locales for v9.10: tree/version-9.10
Locales for v9.03: tree/version-9.03
Locales for v8: tree/version-8
Locales for v7: tree/version-7
It's easy, move all folders from the language pack to website root. After moving files go to Admin Dashboard and open Language Settings then enable your language.