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ConceptNet, at http://conceptnet.io/, is another interesting multilingual resource for exploring foreign language vocabulary. It's described as a semantic network, and incorporates data from various WordNets, Wiktionary, and other sources, and according to its documentation it has a larger vocabulary than WordNet. There are 10 core languages (the usual suspects) with very large vocabularies, but a total of 304 languages are supported to varying degrees. You can do searches from the home page, and there's a Web API that looks fairly easy to add to other tools.

http://conceptnet.io/c/de/eigentlich is a sample of the information on synonyms (including both German synonyms and English and other translations), related words, and word forms returned for one German word. This might be a way to do lemmatization and dictionary lookup together.