Unfortunatly mp3tag cannot compare files to check if they're similar. The only thing you can do is:
Press F3 and enter the following.
"%artist%" HAS "charly" OR "%artist%" HAS "charli"
which searches ARTIST tag for "charly" or "charli"
Then when you want to replace the names you want to change to 'Charly García' use this:
Begin Action Group _Script Test#TEST
Actiontype 5: Format value
Field ______: ARTIST
Formatstring: $regexp(%artist%,'Charly García|Charli Garcia|Charly Garcia','Charly García,1)
End Action Group _Script Test#TEST (1 Action)
The red part Charly García|Charli Garcia|Charly Garcia means find Charly García
the ,1) at the end means ignore case, so if Charly García was actually 'charly garcia' it would still change it.
You can add to this if you want by adding another | after Charly Garcia and then use a different way of spelling the name.
Formatstring: $regexp(%artist%,'Charly García|Charli Garcia|Charly Garcia|DIFFENT NAME HERE','Charly García,1)
and replaces with whats in purple Charly García. It's up to you which one you want to replace and keep at this stage.