"Доктор Айболит говорил по-собачьи."

"Doctor Dolittle could speak in Canine-ese"

Note that по-собачьи would in most contexts be taken to mean "in a dog-like manner," but in a fantasy context, it can be understood as "in the language of dogs." In either case, however, there's no letter -й at the end of the construction.

P.S. In the Russian translations of Hugh Lofting's books, the doctor's surname became Айболит, which literally suggests "Ouch-it-hurts." (Arguably no worse a name for a doctor than "do little"!)