If students need to type special characters, the easiest thing to do is have them setup the special French or Spanish keyboard option on their Chromebooks so they can toggle between a standard US English keyboard and a Spanish or French keyboard. This is especially important if students are needing to enter special characters in a "Locked" Google Form, since they cannot have other tabs/tools open while they are taking the test or quiz. Instructions for how to enable multiple keyboards on a Chromebook are described in the video below:

Use this online Spanish keyboard and then copy and paste the text into a Doc, Sheet, Slide, Form, etc.

Use this online French keyboard and then copy and paste the text into a Doc, Sheet, Slide, Form, etc.

Alt Codes for Typing French on a Desktop Computer