Add language multiselect mutation datatype

[nobears]

See #851

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[Corepex]

@nobears, thanks for your contribution. 🍭 👨‍🚀

I checked out your PR and tried to test it.

I created a data-hub config and added a class with a corresponding languagemultiselect field in the "mutation schema" section.
I tried to add the field to the schema but I got the following error message that this field is unsupported.

Did I forget something - maybe I misunderstood something.