hugolib: Add validation for defaultContentLanguage
hugolib: Add validation for defaultContentLanguage

When `languages` are defined, then `defaultContentLanguage` must match one of those.

Fixes #4298

fedelibre/hugo,gohugoio/hugo,anthonyfok/hugo,fedelibre/hugo,mdhender/hugo,mdhender/hugo,gohugoio/hugo,gohugoio/hugo,fedelibre/hugo,mdhender/hugo,mdhender/hugo,anthonyfok/hugo,anthonyfok/hugo,fedelibre/hugo