A source you may find interesting or useful for this topic is the Open Philanthropy Project’s 2017 Report on Consciousness and Moral Patienthood. Obviously the primary topic of that is different, but it contains some discussion of the general discussion/​sophistication of nonhuman animals compared to humans, including specifically in relation to language (e.g., in this section). Although you may have already encountered the most relevant sources drawn on in that report.
A source you may find interesting or useful for this topic is the Open Philanthropy Project’s 2017 Report on Consciousness and Moral Patienthood. Obviously the primary topic of that is different, but it contains some discussion of the general discussion/​sophistication of nonhuman animals compared to humans, including specifically in relation to language (e.g., in this section). Although you may have already encountered the most relevant sources drawn on in that report.