Use always english labels at first when ingesting vocabularies

Currently, when ingesting a vocabulary that has multiple languages as literals for skos:prefLabel (for the Concepts) and rdfs:label/rdfs:comment (for the ConceptScheme) the backend will only take the first found literal regardless of the language-tag, e.g. skos:prefLabel "Deutsch"@de, "German"@en, "allemand"@fr; will take only "Deutsch" for the concept in MP.

Instead it should take english (if there is a language tag @en) and only if there is no english literal found, it should take the first one.

This issue has some priority because it may compromise the ingest of items (when searching for the label) and it affects the output in the frontend.

It involves: skos:prefLabel for skos:Concept and for skos:ConceptScheme rdfs:label as well as rdfs:comment (but should be good practice for all language string literals.

In general, we may need to think about if we like to support multi-lingual vocabularies.

Opinions? @matej.durco @mkozak @sotiris.karampatakis @lbarbot