Apptionary

Apptionary is a useful utility app designed for those learning a new language or wanting to build their vocabulary in the native language. It collects words you want to save and learn, finds the translations online, and/or allows you to add your own translations. You can add as many translations to one word, in any language. Words can be saved in customized categories for context and quick retrieval. It supports virtually any language, and it synchronizes your list of words and translations to a remote server for backup your content if you have any problems with your devices.

Upgrading removes the ads and allows unlimited words (free app is limited to 50 words)