How to disable shift + quotation mark vowel diaeresis (äëöïÿ) I'm using an external US International keyboard.
When pressing the quotation mark button, together with Shift (⇧ + ") some special mode enables.

When I type a vowel, the following comes out: ä ë ö ï ÿ
This is not my desired output, I wish for "a".

Wikipedia told me this is named Diaeresis:


  Diaeresis
  diacritic
  The diaeresis, also spelled diæresis or dieresis and also known as the tréma or the umlaut, is a diacritical mark that consists of two dots placed over a letter, usually a vowel. When that letter is an i or a j, the diacritic replaces the tittle: ï.  


But several internet searches have not yielded any results in how to change this behavior. 

How can I change this behavior?
What you describe is standard behavior for the "US International PC" input source.  It is intended for Windows users who are familiar with it from that system.  4 keys (` ' " ^) become dead keys for making accented characters used in common European languages.

The fix is to go to system preferences/keyboard/input sources and remove this item and instead use input sources named "US" or "ABC" or "British".

The hardware keyboard called "English International" is the ISO version of Apple's English keyboard. It is used mainly in Europe and has an extra key next to the Z and a differently-shaped return key compared to the ANSI version sold in the US.  It does not require any special input source, all of them work for both ISO and ANSI hardware versions.