Requests to add a new language to a project are not always accepted. The project's Maintainers make this decision, and it may have happened for several reasons.
Here are some common ones:
A project maintainer decided to refrain from supporting that language due to the lack of interest in the product from users who speak that language.
Not enough translators volunteered to translate the project into that language.
Another variant of that language is already in the project. In this case, a project maintainer might want to avoid translating the project to two similar languages, e.g., Danish and Danish (Denmark).
If you would still like to request a language be added or better understand why the request was not accepted, please message the Project Maintainer or Organization Administrator directly.