Download correct crowdin translation branch

[gregorydlogan]

This PR causes GHA to download and apply the correct translations from crowdin. Previously all branches applied the translations from main, which is both defunct and no longer updating with new keys.

[github-actions]

Use docker or podman to test this pull request locally.

Run test server using develop.opencast.org as backend:

podman run --rm -it -p 127.0.0.1:3000:3000 ghcr.io/opencast/opencast-admin-interface:pr-1501

Specify a different backend like stable.opencast.org:

podman run --rm -it -p 127.0.0.1:3000:3000 -e PROXY_TARGET=https://stable.opencast.org ghcr.io/opencast/opencast-admin-interface:pr-1501

It may take a few seconds for the interface to spin up.
It will then be available at http://127.0.0.1:3000.
For more options you can pass on to the proxy, take a look at the README.md.

[github-actions]

This pull request is deployed at test.admin-interface.opencast.org/1501/2026-01-15_21-27-58/ .
It might take a few minutes for it to become available.

[KatrinIhler]

I will not cut an Admin UI release just for this, can you repoint this to 18?