to Dresden:
For Janus Chess there seems to be a notation using J for the Janus. But we are discussing the original Capablanca extended piece set, thus we should not use the name Janus. Traditionally the English names for the new pieces are 'A' for Archbishop and 'C' for Chancellor. Because already a lot of game notations do exist, it will not make any sense to change the English piece letters 'A' and 'C', which name ever might be related to that letter.

In German language we have that problem of taking a first letter for "Kanzler", like in English for the "Knight". Just like that problem has been solved by taking an unused letter N from the word Knight, it will help to take the unused letter Z from Kanzler, which overmore would not conflict with the international English letters. When looking at a bilingual GUI it should be no problem to decide which piece is addressed by a letter. Z for Kanzler would solve this request.

A distinct second problem is to find good names for those pieces corresponding to the already used letters (A+C English, E+Z German). But they should of course be highly related to those actually used.