I hacked into the code and see that SHIFT+CONTROL+1/2 indeed set the characterset to $CC (2) or $E0 (1)
The characters at $CC00 are different from the standaard international set.
Also different mapped to the keyboard-layout.
Now I get the ä etc.
CB73: AD 09 D2 LDA KBCODE [$D209] = $00
CB76: C9 DE CMP #$DE
CB78: D0 05 BNE $CB7F
CB7A: A9 FF LDA #$FF
CB7C: 8D FF 06 STA $06FF [$06FF] = $00
CB7F: C9 DF CMP #$DF
CB81: D0 05 BNE $CB88
CB83: A9 00 LDA #$00
CB85: 8D FF 06 STA $06FF [$06FF] = $00
CB88: 48 PHA
CB89: AD F4 02 LDA CHBAS [$02F4] = $A0
CB8C: C9 E0 CMP #$E0
CB8E: F0 06 BEQ $CB96
CB90: C9 CC CMP #$CC
CB92: F0 02 BEQ $CB96
CB94: D0 10 BNE $CBA6
CB96: AD FF 06 LDA $06FF [$06FF] = $00
CB99: F0 07 BEQ $CBA2
CB9B: A9 CC LDA #$CC
CB9D: 8D F4 02 STA CHBAS [$02F4] = $A0
CBA0: 30 04 BMI $CBA6
CBA2: A9 E0 LDA #$E0
CBA4: 30 F7 BMI $CB9D
CBA6: 68 PLA
CBA7: 60 RTS