'Follower', 'disciple' and so on sound too strong. I want to describe a person with a more casual interest in something. Perhaps similar to what 'amateur' used to mean, before it picked up the sense of being crap at something. So say someone speaks fluent Chinese but they study Japanese on the side out of interest, I might say about them:
They're a Chinese speaker, and Japanese [interested-in-er]
enthusiast (ODO)
noun
1 A person who is very interested in a particular activity or subject:
a sports car enthusiast
syn: fan, … supporter, follower; … informal buff, …
Cited on TFD:
He was as much of an enthusiast in art as I was, and not less anxious to learn to paint. A Tramp Abroad by Twain, Mark
Dilettante
dil·et·tante noun \ˈdi-lə-ˌtänt, -ˌtant; ˌdi-lə-ˈ\ : a person whose interest in an art or in an area of knowledge is not very deep or serious
http://www.merriam-webster.com/dictionary/dilettante
Unfortunately, once you get into these words such as amateur and layman that are less-serious, you do tend to have negative connotations. "Dabbles in..." is a construction I'm fond of, and I was planning to suggest "aficionado" but, as it turns out, that has a more serious denotation than I expected.
