"LANGUAGE" is no longer in .po

[Levin Li]

Celestia/po/zh_CN.po

Line 4265 in 49db030

 #~ msgstr "zh_CN" 

since #1889, this entry has been marked as removed but it is still used in places like

Celestia/src/celengine/dateformatter.cpp

Line 87 in 5b375b9

 const char *orig = N_("LANGUAGE");