ICU-21408 Prohibit alpha4 as language inside tlang.

[FrankYFTang]

Checklist

• Issue filed: https://unicode-org.atlassian.net/browse/ICU-21408
• Updated PR title and link in previous line to include Issue number
• Issue accepted
• Tests included
• Documentation is changed or added

[markusicu]

lgtm

but please also test (& fix if necessary) in Java

[FrankYFTang]

Fixing java would be a lot of work because in C++ LocaleBuilder is based on the UTS35 grammar and parse -t- -u- and -x- and other extension . But in Java right now the ULocale.Builder is based on BCP47 grammar and parse -u- and -x- an other extension and does not parse -t- extension based on UTS35. Could I land this small fix for C++ first?

[markusicu]

Could I land this small fix for C++ first?

yes, ok

[markusicu]

lgtm