Edit ;
I found your old answer via google : https://osticketawesome.com/forums/topic/text-size-in-menus-and-languages-issues/
Hi,
The font size and font weight used in the top menu bar of the agent and admin panel is not readable (see screenshot). Where can I change it in CSS? Thanks!

I'm glad you found the answer, and I hope you've gotten this sorted. But something is weird about the way the navigation bar is showing in your screenshot. It shouldn't look like that. Before changing the CSS, please try clearing your browser cache to see if that helps.
Yes I did, I also cleared the server cache in case. The ticket client side menu is fine, it's only the agent/admin panel that appear so thin and small.
And you are using the fr français (French) language back, yes?
Not that it should matter what language you're using. I'm just trying to reproduce the issue on my end.
Have you been able to resolve this through custom CSS?
Yes, I use the fr pack. I used the css code you provided, but 500 makes it impossible to know which menu you are in at the moment so I'll play with it a bit.
Also, I'm using Chrome.