Bugzilla

@ :flod

I did more digging around and did find a way to still reproduce. Sorry that I did not catch this before you closed the report. 
Even on new profiles if it works to set the language when the profile was just created, restarting the browser and trying to set another language as per the STR in Comment 0 will now fail. 

So new STR would be:

1. Create a new profile
2. Go to about:preferences and scroll down to the “Language” section
3. Click on “Set alternatives”
4. On the popup window that is displayed, click on “Select a language to add…” and select a language of your choosing
5. Click on “Add”
6. Optionally, select the chosen language and move it to the top of the list via the “Move Up” button
7. Click “OK”
8. Notice language is applied to UI
9. Restart the browser
10. Repeat Steps 2->7
11. Notice language is no longer applied to UI

@ :flod

I did more digging around and did find a way to still reproduce. Sorry that I did not catch this before you closed the report. 
Even on new profiles if it works to set the language when the profile was just created, restarting the browser and trying to set another language as per the STR in Comment 0 will now fail. 

So new STR would be:

1. Create a new profile
2. Go to about:preferences and scroll down to the “Language” section
3. Click on “Set alternatives”
4. On the popup window that is displayed, click on “Select a language to add…” and select a language of your choosing
5. Click on “Add”
6. Optionally, select the chosen language and move it to the top of the list via the “Move Up” button
7. Click “OK”
8. Notice language is applied to UI
9. Restart the browser
10. Repeat Steps 2->7
11. Notice language is no longer applied to UI

Happening on all OSes and snap build on Ubuntu 24.04 LTS