The source wants the abbreviation Mar

Not sure. As I'm reading the source, it should (or is trying to) also support "Mär", as set up by the "M${a}r".  $a is defined as

  my(%h)=();
  _Char_8Bit(\%h);
  my($a)=$h{"a:"};
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with the routine _Char_8Bit() mapping "a:" to the char \xe4

sub _Char_8Bit {
  my($hash)=@_;
...
  $$hash{"a:"} = "\xe4";   #   LATIN SMALL LETTER A WITH DIAERESIS
...
}
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In other words, "M${a}r" corresponds to ISO Latin "Mär". So it should work, I think.