Note: Let's also remember that although the code is still there in installation/src/, we can't choose admin and site default langs, multilang data (I am finishing now a specific multilang sampledata plugin to cope with that), etc.

It is working for me as of f180ef0

Have you installed the language as I did when installing a clean Joomla at Joomla install time, not from administrator install languages?

Yes, performed exact steps as in your video.

Hmm... It works now... Closing

@infograf768 could it be language related - I am sure that I had something similar the other day. Content language ws only created for some of the languages

can't be language pack related if it works once and not always. I guess maybe this test site had some ancient code remaining somewhere. Maybe a js conflict. We have only 3 languages that we can install and all have the correct 3.8.8.1 code.

I have no idea what languages I selected when I tried and failed :(

#16337 could be related?

Unrelated. @mbabker managed for us to be able to install/use fr-FR, de-DE and fa-IR on 4.0.
There never is an issue when installing with Install Languages in back-end, only a bit unstable when installing Joomla itself. I mean the language installs but Content Language may or may not.