My grandmother has alzheimer's disease, so will she forget english and relearn the german of her childhood?

Yes & No: While no one can predict the future per se (aside from death & taxes), those w/dementia tend to forget recent events while remembering distance memories. So while it is possible someone might lose their "english as a second language" skills, they won't so much "relearn" their mother tongue as revert back to using it. Hope that semantic/philosophical answer makes sense & helps.

Possibly: It depends on how old she was when she switched to the new language as well as how verbally oriented she was.

It may happen: People with alzheimer's disease often lose their second language and revert to their first one. It does not require relearning, they just remember it well.