Teaching Multiple Subjects I plan to do my PhD in Computational Linguistics in the near future. As one would imagine, it requires a bit of knowledge in Computer Science as well as Linguistics. Particularly, I enjoy French and Japanese.

The problem is that I want to teach too many topics at the university level. In my imaginary world, I would teach courses in Computer Science that overlap with Computational Linguistics (e.g. Structure of Programming Languages, Artificial Intelligence, etc.), as well as French and Japanese.

As far as I know, we have only one professor at my university who overlapped topics - the Russian department needed a temp, and one of the Computer Science professors was a Russian native; he filled in for a semester.

Presumably I would only teach courses that overlap with my research (the Computer Science courses I listed, French/Japanese Syntax/Morphology, etc).

Granted this is pretty specific to my case, so how about I generalize: how plausible is it to teach multiple subjects that are inter-departmental at the university level? If it is indeed plausible and is something that I can specifically work toward, what steps are necessary to be able to do so?

Edit: To clarify, this would be teaching as a postdoc, not particularly during my PhD. I think my brain would explode if I tried to take on that kind of workload.
As someone who is doing my PhD in Natural Language Understanding I first of all recommend you are careful what you wish for. Undertaking a teaching post while doing a PhD is very time consuming and I have found that it has got in the way of my research. So much so in fact that I limit myself to no more than 6 hours of teaching a week. 


  In my imaginary world, I would teach courses in Computer Science that
  overlap with Computational Linguistics (e.g. Structure of Programming
  Languages, Artificial Intelligence, etc.), as well as French and
  Japanese.


I'm sure it's different depending on the university, but whatever department is paying your PhD stipend most definitely has first call on where and what you teach. Although I'm a mathematical linguist, I spend all of my time teaching programming and artificial intelligence. Another reason for this is because my background is mathematics and computer science. 


  Granted this is pretty specific to my case, so how about I generalize:
  how plausible is it to teach multiple subjects that are
  inter-departmental at the university level?


Taking on a PhD project is about becoming a specialist, not a generalist. Everyone is arguably a generalist. If you are a Computational Linguist, just take on modules such as Intelligent Systems, Computational Linguistics, Mathematical Linguistics, Logic Programming, Math for Computer Science, etc.


  If it is indeed plausible and is something that I can specifically
  work toward, what steps are necessary to be able to do so?


I think it is possible. However, you would need to get explicit permission from your project advisors which I imagine would be difficult. You would then have to discuss how you can help with the other departments such as Linguistics. However, be prepared for rejection if you are not classically trained in the subject and hold some sort of academic merit.

When you start your PhD you will not become a lecturer most likely. You will be encouraged to take on a teaching assistant or demonstrator role, aiding a lecturer. This is still good as it's the next best thing and sometimes you can get loads of marking. It will also give you loads of time to reflect and decide what you want to teach.