I believe that this film rendered the book quite faithfully.
I think you're right, you're the second person who has said this to me and I'm ashamed to admit I haven't read the book, but I find myself often put off by iconic books translated into English from the language they were originally meant to be told in. In the case of the Motorcycle Diaries, it was originally Spanish, but my friend who reads/speaks Spanish reads it in Spanish and said exactly what you said.
Oh, and I simply adore Gael García Bernal.
He is a great actor, and I most recently saw him in an odd comedy-type drama called 'Mozart in the Jungle' where he plays an idiosyncratic genius conductor. I know that he is famous for much better roles than this, and is one of Mexico's finest actors but for some reason, that memory of binge-watching Mozart in the Jungle just popped into my head 😂