ForeignLanguages are languages that are not recognized as the official language in a given part of any country.
This seems to me the only definition really applicable to the situation of French in the Dutch speaking part of Belgium, of Catalan outside the Catalan region, etc.
Alternatively, any language which is foreign to the location in question. Such as English in some parts of the DeepSouth? (see also YouMightBeaRedneck?).