The Fritz!Fon is automatically retrieving the language settings from the connected Fritz!Box. In other words, the Fritz!Fon is using the same language as the Fritz!Box. If the Fritz!Fon is connected to a Sunrise Internet Box its using the default language (German) and has unfortunately no possibility to change the menu language.
For non-German Speaking people customers with a Sunrise Internet Box Sunrise is recommendig to use a Gigaset. The Gigaset is using the Cat IQ 2.0 standard and offers to change the language on the phone.