Changes 1/3 of the strings...
I replaced "LANGUAGE LANG_ENGLISH, SUBLANG ENGLISH_US" with "LANGUAGE LANG_FRENCH, SUBLANG_FRENCH", too.
Place into %ProgramFiles%\CCleaner\Lang and overwrite the existing file. Or %ProgramFiles(x86)% in 64-bits Windows. Or in [custom CCleaner location]\Lang if you changed the default location.