OK, so there is no textivate-specific way to add accented characters or special characters to textivate.
Note that most textivate exercises DO NOT require the student to type in accented characters. The only exception to this is the Gap-fill (and user-defined Gap-fill) activity, and only when no word list is provided.
But as a teacher creating your own resources, you will want to make sure that all the special characters are included in your resource.
If you are using a smartphone or a tablet, there shouldn't be a problem inserting accented characters either, as in most cases it's simply a case of long-pressing the un-accented version of the character and then selecting from one of the accented options.
If you're using a desktop computer, laptop, or notebook, it can be a bit trickier.
Personally, I use a Spanish keyboard, which makes it s-o-o-o much easier, but if you don't have that option, read on for solutions for Google Chrome and Mozilla Firefox.
Google Chrome - "Special Characters - Click and Paste"
This Chrome extension is a really neat solution to the problem of inserting special characters for French, German and Spanish. You can get capitals by using the shift key. And it does things like upside-down question marks and exclamation marks ¿¡
It adds an á icon to your chrome browser, and clicking this gives you a list of accents to choose from. Click the desired character and then paste into the text box on textivate (or use the ctrl+v shortcut).
Mozilla Firefox - "abcTajpu"
Install this add-on to Mozilla Firefox, then add accented characters by right-clicking into any text box, you should then see an "abcTajpu" context-specific option, which gives access to hundreds of accented and special characters.
You can modify the add-on via the add-on options menu, to add the letters for the languages of your choice. For example, if you add French and Spanish, your context-specific right-click will produce a list with "abcTajpu" (ie. the default), plus "French", plus "Spanish".