The easiest way would be if, as DrPoodle said, you write a script to do it. Or if there is a mass find and replace tool that searches in many files (one that does PHP may be hard(er) to find than simple text files, but I'm not sure).
Also, the buttons would need a change, but I think there is only one image that contains the word "Thread"
There is an image .psd file which will make changing the image virtually a piece of cake and is available at these forums, posted by Chris Boulton - shouldn't be difficult to find.
As you said, it is a lot of work. Is it necessary? Surely you/your members can adapt to seeing threads instead of topics.
You also need to keep in mind that any future MyBB release that makes changes to the language files (which happens occasionally) you will need to re-modify the files accordingly.
You'll probably find yourself asking, is it worth the trouble?