Good morning !
I understand, being a developer myself I know that it can be complicated.
I don't know how your application is made so I won't allow myself to judge, I just wanted to raise the problem just in case!
Afterwards there is perhaps a troubleshooting situation while waiting for the modification? You can't detect the /fr of the current window url and manually redirect the language? (This is just a possible proposition, but of course it depends on the application).
In any case good luck !