I have only bought C# packages, but it really depends on the content. If there was something I really needed/wanted that was JS-only, I would probably still get it, but it would significantly impact the decision to buy, as it would make it harder to read/edit. I don’t think I would ever buy a Boo package though, but that’s personal preference.
I wouldn’t want to mix languages in my projects, but only because of consistency therefore reliability. At the end of the day, Unity is doing the same stuff with all of them. However, if there was a package for sale that used mixed languages in it’s functionality (instead of just providing the same code in all languages, for example) I would probably avoid it, because it would make me question the consistency of the programmer that wrote it. I want to be able to edit the code files I buy, so although there’s nothing wrong with mixing languages, it would suggest that the programmer’s own practices are far enough out of line with my own that I wouldn’t want to use it
I am experienced enough of a coder that the choice of language is nearly irrelevant to me. For my own stuff, I use Boo. The only annoyance is in Unity when something has to call across from one language to another, to arrange the files so they compile in the proper order. Most separate assets don’t need to do this, so I don’t really care what language an asset author chose when I buy one.
If I want an asset I don’t much care about the language. But I appreciate it when assets have both C# and Unityscript as sometimes it is a help when communicating with another plugin that is only written in C# or Unityscript.