Brithenig
Brithenig is an invented language, or constructed language ("conlang"). It was created as a hobby in 1996 by Andrew Smith from New Zealand, who also invented the alternate history of Ill Bethisad to "explain" it.
Brithenig was not developed to be used in the real world, like Esperanto or Interlingua, nor to provide detail to a work of fiction, like Klingon from the Star Trek scenarios. Rather, Brithenig started as a thought experiment to create a Romance language that might have evolved if Latin had displaced the native Celtic language as the spoken language of the people in Great Britain.