Content Translation depends on user testing for new languages using Beta Cluster. To make sure user and developers can test well, we need to have all language wikis that Content Translation supports right now (and more as we grow).
We also need to effectively test the redirects to target language wikis, publish in target wikis, beta feature propagation etc Since, Beta is not exact configuration as production, issues like, https://www.mediawiki.org/w/index.php?title=Topic:Scfdmyjhftringtd tends to happen.
As per documentation, process is described at: https://wikitech.wikimedia.org/wiki/Nova_Resource:Deployment-prep/Add_a_wiki