In the additional letters, the small ñ is missing (Capital Ñ is in the list). It's a very used letter in Spanish. (Additionaly, the Ç and ç are missing they are used in Portuguese.)

I don't know how this can be solved in the general case. Maybe add 10 free letters and ask the user to supply the unicode number, or type/copy the letter in a text field of the web page.