Question: I've been reading many opinions from the United State Supreme Court and discovered the phrase "inter alia," meaning "among other things." I have not encountered this phrase outside of these opinions.

Is the phrase "inter alia" primarily localized to legalese? Or would it be acceptable to use it in general writing?

Answer: 
The best course, undoubtedly, is to use "among others", a phrase that can refer to people or things. 

It should be noted that "inter alia" is Latin for "among others" when "others" are things. If the others are persons, "alia" must be changed to "alios" (the OED quotes, from 1670, *The lords produce **inter alios** John Duke of Lancaster*); but when persons are meant, it is much better nowadays to use English.

Reference: A Dictionary of Modern American Usage, *B.A. Garner*, A Dictionary of Modern American Usage, *H.W. Fowler*.
It is certainly used more widely, but it says nothing that *among other things* doesn't say. The English version will be understood by all readers, and removes the risk that you will be perceived as someone who likes to show you know a bit of Latin.