i18n and relation type label

Yes, that's currently correct.

The relation label is just generated using something along the lines of "$endpoint_one $relation_type_label $endpoint_two. I don't know if that would be easily internationalisable. The relation type label and reverse label should definitely both be internationalised. But the label isn't enough, because presumably someone will want to translate the label to a language that doesn't use the form "subject verb object", and also the label is currently useless for unary or n-ary relations with n>2.

So yes, let's get the relation type label and reverse label translatable.

Relation labels are more or less useless in most current situations except the natural language formatter which needs an overhaul. See #1479346: Create a display formatter to display relation_endpoints with tokens