Mycroft, having noticed Ilde's partial deafness from their first verbal exchange, had chosen to continue speaking verbally due to Ilde's proficiency at lip-reading and the assumption that she could hear him in some capacity (her study of music would seem to attest to that). Additionally, she wouldn't ask someone she didn't know to translate a book aloud if she wouldn't be able to understand them. However, it seems she's having more trouble than she anticipated, intent as she is upon trying to watch Mycroft's words and learn the German at the same time.

It's not a sure thing, but Mycroft may have a solution.

"Do you speak BSL?" he asks in British Sign Language. "Or ASL?" This in American.