[ENHANCEMENT] Allow finer rules for missing languages and missing label items

[neilfromcork]

Rules for language plugins should be as follows:

• If using the default language and a label value is not found then return the label key
• If using another language and a label value is not found, first check the dictionary of the default language. If found, return it, if not found, return the label key
• If a request is made in a non existing language then use the dictionary of the default language

[liamie]

This seems to be operating correctly for Irish defaulting back.