Is it only for Japanese?

No. What you can look up is decided by the dictionaries you import — Fushi reads Yomitan, MDX (MDict), DSL and StarDict dictionaries, its deinflection covers many languages, and English lookup also resolves irregular forms and phrasal verbs (gave up → give up).

The recommended pack contains Japanese dictionaries plus Japanese / English pronunciation libraries; learning something else, just skip it. "Download dictionaries" in dictionary management lets you pick ready-made dictionaries by "learning language" and "your language" and download them in one go, or you can import dictionaries you already have — see how to import dictionaries. The interface itself comes in 17 languages.