Brithenig
Brithenig is broadly considered one of the best constructed languages on the web. It started as a thought-experiment to create a Romance language that might have evolved if Latin speakers had been a sufficient number to displace Old Celtic as the spoken language of the people in Great Britain. The result is a sister language to French, Spanish 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 Old Celtic and from English throughout its 'pseudo-history'. Although other Romance languages have Celtic influences, none of them are so thoroughly influenced as Brithenig.Brithenig was created in 1996 by Andrew Smith from New Zealand, who also invented the alternate history of Ill Bethisad as its home.
