It would if I wouldn't replace the French menu, too

Actually, what I'm doing is
1 replace GB_F.CSF with USA_F.CSF
2 replace F_F.CSF with USA_M.CSF
3 replace both Lng_English and Lng_Francais with Lng_American
4 change the splash screen logo

No. 1 and 2 are easy since these are seperate files under \V_2\Voices. No. 4 can be done using GifExtractor, but no. 3 is kinda tricky since you've to replace the files within the navboot file itself. Replacing is not that big deal using XVI32 Hex Editor, but the language files contain a header with their own filenames in it, meaning just replacing Lng_Francais with the contents of Lng_American isn't enough.

So, for no. 3 I'm using ambishop's split.exe to get the sole Lng_American file. I create two copies of it named Lng_English and Lng_Francais and then I use XVI32 to change the filename of the corresponding headers. The last step before placing back the new files into the original navboot file is to correct the CRC24 checksum. This can be done using crc24.exe from the HackTheIBus forum.

That's about it...