With the exception of ancient languages, of which it is rather difficult to know how to pronounce it, knowing something about any other language would probably mean being able to use it. I would almost say, that knowing something factual about a language, for example which words should be used in certain situations is more cultural knowledge than language knowledge. I would think calling interest in the grammer of a language, having an interest in a language. But I wonder if that would be a right assumption. Because a language interest can have many different sides. Grammer and cultural use being only two of them.

If you look at it that way, that language interest is much brother than speaking it. I find it interesting, that languages take words from other language if they can use them. Especially the English language is strong in this sense. Many common words come from the Normandian occupation. Many of its boating terms are originally Dutch and so on. But on the other hand I would sooner call this a cultural habit of the English, than a habit of the English language. But all this IMHO off course.

In the end an interest in language can be in the history of, in grammer, in use, in application (as with computer languages), in meta-language, etc.

IMO, having 'languages' as an interest is different from speaking them. Actually, it could include it :)
I'm interested in ancient languages, computer languages, code languages... yeah, there are sooo many languages =) I'd like to speak Hebrew fluently too ...

I must admit I find this a strange interest. But that is probably because it is so obvious and pervasive in daily life. I would never have thought of it as an interest. But as I ponder about it, I find it is more important than any of the other interest I have. Because without language we would not be able to achieve what we have achieved. So language and communication are two important activities and interests humans know and have.

So what else can be said about languages? I speak three and understand a few more a little bit, although that little bit would probably not save my life if it was necessary.