After about an hour, the letter disappears.

A new one reappears right in front of Matilda some ten minutes later.

Dear Jennifer Honey,

The letters are the least intrusive way to communicate, but we can also summon you in addition to Matilda. We are acquainted with a witch who has agreed to give her lessons; scheduling may prove challenging if your days are not the same length as ours, which seems likely. Language lessons are also possible. We may be able to secure the help of a person with the native magical ability to speak any language to help with that.

This is again signed in Ertydon characters.