To type a diacritical mark (such as an accent mark or umlaut), you can tap and hold on the base key to show more options. For example, hold down on the A-key for Ä or Á.
To type a symbol such as an apostrophe or hyphen, you will find these by tapping the 123-key on your keyboard on mobile. This will open a keyboard of numbers, punctuation marks, and special characters.
Many European languages use letters with diacritical marks, such as the umlaut in German, the cedilla and circumflex in French, and the tilde mark in Spanish.