GOOGLE SHEETS: Adding letters to the end of words, replacing specific letters with other letters (Concatenate, Regexreplace, Textjoin?)

Question

PLEASE REFER TO THE FOLLOWING SHEET: https://docs.google.com/spreadsheets/d/1gz5VHxyn3L2-Y_TQwSrp6LryIvTFxl3N17Da7LSzDaA/edit?usp=sharing

I've used CONCATENATE in column C to add the letter(s) in column B to the end of the word.
Additionally, I want column C to change any of the letters a, o, u to their umlaut form (ä, ö, ü).
In the case of vowels that are together, umlaut only the first vowel (see example in C18: "Häuser").

You may note that M2a words don't add an umlaut.
So, I'm thinking that the formula should reference column D to determine both the word ending and umlauts.

BELOW ARE EXAMPLE OF HOW THE WORDS SHOULD CHANGE:
Uhren changes to Uhren
Doge changes to Döge
Fluge changes to Flüge
Hauser changes to Häuser
Blatter changes to Blätter

Answer 1

Here's one approach you may test out:

=let(x,{"a";"o";"u"},y,{"ä";"ö";"ü"}, index(map(A2:A,B2:B,D2:D,lambda(ax,bx,dx,if(dx="",,let(a,join(,{ax,bx}), b,mid(a,sequence(1,len(a)),1), 
 join(,map(b,hstack(,choosecols(b,sequence(columns(b)-1,1,1))),hstack(,choosecols(b,sequence(columns(b)-2,1,3)),),
 lambda(c,d,e,ifs(dx="M2a",c,or(sum(countif(x,{d,e}))=2,countif(x,d)),c,1,xlookup(c,x,y,c)))))))))))