Will Lingo support languages other than English?

Yes, and no.

I have absolutely no problem with adding support to Lingo for your language, provided you provide me an on-line (REST) API to a dictionary source for your language that is legal to use for an open-source project, and obviously does not cost money.

I've found that dictionary APIs are very hard to come by. I don't know of any non-English API that fits the description above.
If you happen to find one, by all means report a bug titled "Use X API" or something similar. I'll do my best to see about implementing it.

This said, Lingo itself is still fully i18n'd via gettext. That is, the actual text of the Lingo interface will be in your desktop's OS, even if the results are for English words.

In other words, it's probably best to think of Lingo as an English language dictionary that is usable by people in their native languages.