Operation Unz̖̬̜̺̬a͇͖̯͔͉l̟̭g͕̝̼͇͓̪͍o̬̝͍̹̻
JavaScript (Node.js), 45 bytes



s=&gt;s.replace(/([aeiouy]̈)|[̀-ͯ҉]/ig,'$1')


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More readable version:

s=&gt;s.replace(/([aeiouy]\u0308)|[\u0300-\u036f\u0489]/ig,'$1')