add note about how missing keys are treated

customize.dist/translations/README.md

@@ -107,6 +107,9 @@ define(function () {
 It's important that you modify just the string, and not the variable name which is used to access its content.
 For instance, changing `_languageName` to `_language_name` would make the string `'Pirate'` inaccessible to the rest of the codebase.
 
+If a key is not found in your translation, the default English version of that key will be used.
+This is to make sure that buttons and other interface elements are not empty, but it's obviously not ideal.
+
 ## Verifying Your Translations
 
 It's advisable to save your translation file frequently, and reload Cryptpad in your browser to check that there are no errors in your translation file.