How do I correctly map letters in the database?

I have two tables. One table with the letters of different countries and a second table with a mapping of these letters to each other.

I need to make a query to get the mapped letters of the two languages. Can you tell me how this can be done optimally?

The letter table

id	letter	language
1	A	en
2	Ä	de
3	A	de
4	O	en
5	O	de
6	Ö	de

The letter mapping table

id	letter1(letterTable.id)	letter2(letterTable.id)
1	1	2
2	1	3
3	4	5
4	4	6

Would it be better to create a separate table for each alphabet?

Maybe there is some other architectural approach for this kind of letter matching? I would really appreciate it!

Answer

This can be achieved with a join that is restricted to the two languages you want to check:

select en.id as id_en, 
       en.letter as letter_en, 
       de.id as id_de,
       de.letter as letter_de
from letter en
  join letter_mapping lm on lm.letter1 = en.id
  join letter de on de.id = lm.letter2 and de.language = 'de'
where en.language = 'en';

Online example