Wikipedia
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.
The result is a sister language to French, Catalan, Spanish, Portuguese, Romanian and Italian, albeit a test-tube child, which differs from them by having sound-changes similar to those that affected the Welsh language, and words that are borrowed from the Brittonic languages and from English throughout its pseudo-history. One important distinction between Brithenig and Welsh is that while Welsh is P-Celtic, Latin was a Q-Italic language (as opposed to P-Italic, like Oscan), and this trait was passed onto Brithenig.
Similar efforts to extrapolate Romance languages are Breathanach (influenced by the other branch of Celtic), Judajca (influenced by Hebrew), Þrjótrunn (a non-Ill Bethisad language influenced by Icelandic), Wenedyk (influenced by Polish), and Xliponian (which experienced a Grimm's law-like sound shift). It has also inspired Wessisc, a hypothetical Germanic language influenced by contact with Old Celtic.
Brithenig was granted the code BZT as part of ISO 639-3.
Andrew Smith was one of the conlangers featured in the exhibit "Esperanto, Elvish, and Beyond: The World of Constructed Languages" displayed at the Cleveland Public Library from May through August 2008. Smith's creation of Brithenig was cited as the reason for his inclusion in the exhibit (which also included the Babel Text in Smith's language).