A posteriori conlang
An a posteriori conlang is a constructed language whose vocabulary or grammar is based on one or more already existing languages. A posteriori conlangs are the opposite of a priori conlangs, which are languages made entirely from scratch. Usually a posteriori conlangs will bear a strong resemblance to their "mother" languages (for example, Alphistian looks a good deal like Dutch).
What percentage of conlangs are made to be a posteriori is currently unknown. If you're curious to see more examples of a posteriori conlangs, click here.