EuroWordNet is a system of semantic networks for European languages, based on WordNet.
[1] Each language develops its own wordnet but they are interconnected with interlingual links stored in the Interlingual Index (ILI).
The original EuroWordNet project dealt with Dutch, Italian, Spanish, German, French, Czech, and Estonian.
Access to the full database, however, is charged.
In some cases, OpenThesaurus and BabelNet may serve as free alternatives.