The easiest way to add contacts to Address Book is by clicking the star next to a contact in the Header Pane of a message. But if you are creating a contact directly, and the name or address contains characters not readily available on the keyboard, copy the characters with Ctrl+C, then paste them into a New Contact window with Ctrl+V, or right-click in a contact box, Paste.

As an aside, there is an add-on that enhances the insertion of characters and symbols when composing messages:

https://addons.mozilla.org/en-us/thunderbird/addon/abctajpu/

Thank you for your answer, sfhowes. Firstly it concerns Win 7 and Thunderbird version 52.3.0 (32-bit). I could not find any star next to to a contacts in the Header Pane of the message. Please attache a screen capture. abctajpu looks interesting, But it seems to be rather overkill for my purpose. I'll continue with the old good Copy/Paste technique. It is pity that when you edit/create a contact and you put Notes you can not select language. It is a common text after all.

In the attached picture, note the grey star next to the Reply-to address: clicking the star will add the contact to Personal Address Book and the star will turn yellow. Clicking a yellow star, such as for the From address, allows you to edit the contact details and move it to a different address book. Right-click an address for these and other options.

Thanks for your answer. The clicking on the star leads to editing of the contact and back to the original issue with typing of "national" characters. It will be copy/cut/paste for me as I always did. Another way is to misspell the word and let the spell check do the work. I'm sure you understand. Still it cannot be used in the editing contact, only when composing a message. I cannot see that we can go further with this problem.

Another vote here for abcTajpu. If it does the job it is not overkill.

I really miss it in environments where I cannot use Thunderbird or Firefox.