Is it possible for any constructed language, be they conceived for hypothetical historical outcomes (Brithenig, Wenedyk, etc.) or for international communication (Esperanto, Interlingua, etc.) or even some alternate conlang to become the official language of a nation or province? POD is whenever you want.
Hard Mode: The conlang is successfully used, or the most commonly spoken in said area.
Nightmare Mode: The conlang is generally the native language of most speakers.
Heaven or Hell Mode: A priori languages. (IE: Klingon, Dothraki)