An interesting data point for putting a ceiling on an animal's ability to produce language is this lecture discussing various animals (mostly apes) learning American Sign Language (starting at ~1h18m).
My take-away is that some chimps may be capable of remembering and reproducing words, but not string them together meaningfully: word order is effectively random, length of sentence does not correlate with information content. The lecture discusses (at about 1:34:56) the turning point of the field when H. Terrace was unable to reproduce the generally optimistic findings, e.g. about Koko using "syntax", when teaching his own chimp American Sign Language and then published Why Koko Can't Talk.
If the above is correct, my prior would be for dogs to be at or below the level of chimps - which would still be an interesting finding. What the history of the field highlights, however, is that many humans really want animals to be able to speak AND humans are great at pattern matching meaning into arbitrarily strung together words, which decreases my confidence in anecdotal evidence without more rigorous studies.