Hm.

All I did was swap the filenames of the french and british voice files...There's several sets, and I may have just missed one or two. That may be the issue that's causing my bug.

...then in NavTweak, I just export the lng_american, use a hex editor to change the header in the file to lng_francais (so that navtweak would allow me to replace the existing lng_francais with it), correct the CRC, then replace lng_francais...