Concerning Grendel's attitude toward language in the novel, Grendel, remember that according to the world of this novel, according to the conventions used in the novel, the narrator, Grendel, writes after the events of the novel have been experienced, at least most of them. The present of the novel is the 12th year of his battle with humans, even though most of the novel is history or background. Thus, Grendel relates the history of his encounters/experiences with humans and their/his language. The narrator's attitude itself doesn't change throughout the novel, though the novel is in part a record of how his attitude toward language changed in the past. The novel is a construct written by the narrator to relate whatever it is he wants to relate, and part of that includes his changing use of language.
Remember, too, that Grendel is an unreliable narrator. He writes explaining and arguing for specific philosophies. (The novel is kept from being didactic or preachy by numerous means, however--one of the most important being that Grendel loses.)
All that said, the most important element of Grendel's attitude toward language is the love-hate relationship he has with it.
The Shaper recreates reality with lies, according to Grendel, and lies involve the use of language, of course. Truth means nothing to the Shaper. He turns losses into wins. He gives hope where there is none. He slants and turns the truth to form his lies. Unferth, too, spouts untruths with his bold words, and the humans as a whole sing at funerals, turning what should be sad, hopeless occassions into celebrations.
The reader isn't told how it is that Grendel can understand the humans and can himself speak and use language, and humans are surprised when they hear him actually speak. This highlights the subject of language in the novel, brings attention to it. As mentioned, Grendel thinks little of the human uses of language.
Simultaneously, however, Grendel also relates his own attempts at becoming an artist. In fact, the novel itself is his crowning artistic achievement. It is the "monster" as artist. It is his imitation, or in his view, his improvement on what the Shaper and other humans do.
Specifically, in addition to writing the novel as a whole, Grendel first shows the reader his early attempt at poetry. He then demonstrates his improvement as a poet, revealing later in the novel poetry of a much higher quality. He writes one chapter as a screenplay. He is an emerging artist.
Grendel draws attention to his language and his art throughout the novel. He is witty, and he is proud of it. He, the supposedly stupid monster, is clever, humorous, creative, and highly intelligent, as he demonstrates with his use of language.
The novel, at its center, is of course about reality vs. art. Grendel's philosophy comes down on the side of reality, and humans on the side of art. Yet, echoing his philosophical beliefs, Grendel uses art to entertain himself and somehow survive the tediousness of existence.