Learners of Welsh will probably know about the “To Bach” program from Interceptor Solutions that you can use to help typing the special characters you need in Welsh.
(It’s Windows-only I’m afraid).
Well, it can already be helpful in some other languages, and you can also configure it to add extra characters of your own choice. It’s a bit fiddly, but do-able. Just make sure you back up the configuration file.
I’ve put some instructions for German and Scandinavian here: