François Pénzes: My "thoughts" are that some developer(s) put in a great deal of effort to modify your copy of Uftree.exe version 2.5 to give you a French Language user interface, and that you replaced this version when you upgraded to UFT 2.8. It appears that version 2.8 was available only in English.
If you re-install a lower numbered version of UFT, even in a different directly, it is likely that the Windows Registry will be redirected to that version only. It may be that you can then re-install UFT 2.8 in another directory (folder) and have both versions operating off the same database.
It may be advisable to back up everything twice before attempting this.