I found a solution, but need to edit some game files.

1:Open game path "X:\...\Steam\steamapps\common\Far Cry 6"

2:Open folder "data_final\pc\worlds"

3:Search "english" and should find 20 files, back up them and deleted from this folder

4:Search "mexican" and copy them, rename "mexican" to "english"(e.g. "fc6_lem0020_mexican.dat" to "fc6_lem0020_english.dat")

5:Rename all 20 files, done!

(If you want to switch back to english, use backup to overwrite the 20 files in game folder, or a simpler way, verify integrity of game files)

I just found out this solution, maybe there are some bugs I don't know, but I played for 5 minutes, the game runs normal without crash.

P.S. I also tried some files with "spanish". It sounds like unfinished with machine dubbing. Therefore, in the Steam Store page, this game doesn't support Spanish(Europe) audio.

Thanks to:
https://steamproxy.com/id/VANDaDa