Current version has support for english and russian versions of LJ interface, if you use LJ in another language, the script's button will always be visible. To add support for another language, post a comment with LJ's translation of "Friend of:" string (if possible, give me a long enough string which occurs in both "Friend of" and "Also Friend of" strings, if they are completely different, give me both of them).
If you don't like this script — don't use it. The data it uses can be retrieved from LJ's fdata.bml file (which is what this script uses) and from the directory search (paid users only). It can even be retrieved by lots of googling, so I don't think that this script somehow violates your privacy.
upd (24.01): script updated (changed address of userinfo pages).