libintl: LANGUAGE env var doesn't accept multi language designation.

[Takehiko NOZAKI]

GNU libintl can designate multi language to search nls by language list separated by ':' in LANGUAGE environment variable. https://www.gnu.org/software/gettext/manual/html_chapter/gettext_16.html#The-LANGUAGE-variable

but Citus libintl does'nt accept such format.