Simplify the following text for a Grade 6 native English-speaking person.
Original: Zamenhof , the inventor of Esperanto , created an `` h-convention '' , which replaces Ä , Ä , Ä , Ä , Å , and Å with ch , gh , hh , jh , sh , and u , respectively .
Simplified: Those letters Ä Ä Ä Ä Å Å are usually not found on mobile phones or keyboards .