The problem is that 10tetex.cnf in /etc/texmf/language.d should be named 00tetex.cnf. Otherwise, update-language does not work. So first you rename the file, then 'sudo update-language'. After that, you must run 'sudo fmtutil-sys --all'. I don't know why update-language does not run it because update-language is useless without running fmtutil-sys.
The problem is that 10tetex.cnf in /etc/texmf/ language. d should be named 00tetex.cnf. Otherwise, update-language does not work. So first you rename the file, then 'sudo update-language'. After that, you must run 'sudo fmtutil-sys --all'. I don't know why update-language does not run it because update-language is useless without running fmtutil-sys.