I want to find the location (or source) of the translation of the 'All [CATEGORY]' string in the submenu (All componenten in this situation):
Are you talking about a submenu in the main navigation? If so the string is defined in
lib/web/mage/menu.js on line
You can translate it by adding:
"All ",Your translation ,lib,lib/web/mage/menu.js to your translation file.