5643: Update french.txt r=def- a=Sehyn
Added translation for some untranslated strings.
Fixed a translation mistake -> "Reconnecte dans %s is Reconnexion dans %s"
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## Checklist
- [ ] Tested the change ingame
- [ ] Provided screenshots if it is a visual change
- [ ] Tested in combination with possibly related configuration options
- [ ] Written a unit test (especially base/) or added coverage to integration test
- [ ] Considered possible null pointers and out of bounds array indexing
- [ ] Changed no physics that affect existing maps
- [ ] Tested the change with [ASan+UBSan or valgrind's memcheck](https://github.com/ddnet/ddnet/#using-addresssanitizer--undefinedbehavioursanitizer-or-valgrinds-memcheck) (optional)
Co-authored-by: Arthur.B <48888351+Sehyn@users.noreply.github.com>
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