Steps to reproduce the issue

with 5.1.0 Alpha 2 or the current 5.1 branch on github you can not install or update any languages.

This is because of this line

<targetplatform name="joomla" version="5.[0]"/>

There are several problems here

1. The first is the obvious one that the targetplaform needs to be updated
2. The second is that as far as i can remember you have always been able to install a language for an older release if there is not a current one available. That is why we display the following message and do not disable the install button or the re-install button

but if you try to install it then you get the wonderfully helpful error message

3. If you had language XXv5.0.1 and update Joomla to 5.1 then even if there is a language update XXv5.0.2.9 you can not update to it. But you will still be reminded to update

And the slightly more helpful error message

Summary

Language files are not the same as extensions. There is generally no serious problem if the only one available to uise if from an earlier release.