Please add the language codes rm-rumgr, rm-surmiran, rm-sursilv, rm-sutsilv, rm-vallader, rm-puter to the list of language codes supported for monolingual text values.
In T210293#5155124, @Lea_Lacroix_WMDE wrote that I should file a separate ticket for this. Also pinging @WMDE-leszek on this ticket since it appears to be controversial.
Usage example: Try adding the sentence “Ils tgauns vivan dalla naschientscha naven ensemen cullas nuorsas.” as usage example (P5831) in language “rm-sursilv” for lexeme tgaun (L45642). Currently, this does not work; see screenshot.