In WikiData for monolingual text datatype, is required the message
datatypes-type-monolingualtext
that seems does not exists.
This is used in Special:ListProperties and in Property namespace; I don't know if also in other page.
In translatewiki.net exists the message
Datatypes-type-monolingual-text/en
but I don't find it in WikiBase repository (maybe it was deleted in past).