In WikiData for monolingual text datatype, is required the message
that seems does not exists.
This is used in [[ https://www.wikidata.org/wiki/Special:ListProperties?datatype=monolingualtext | Special:ListProperties ]] and in Property namespace; I don't know if also in other page.
In translatewiki.net exists the message
but I don't find it in WikiBase repository (maybe it was deleted in past).