What is {gv-GB} in language setting?

In LibreOffice 6.0, one of the options at Tools>Language>For selection (or Page or All Text) is {gv-GB}. What is this?

That’s for Manx, for more, see Languages · TryGhost/roon-i18n Wiki · GitHub

Thank you.
It seems quite odd that Manx would appear by default. Has there been a population explosion there?