Every lesson helps me understand American culture and its seemingly easy language.
I see no reason why you'd have to substitute "straightforward" for "easy" in that sentence. You could use "straightforward," but I think "easy" works just as well. There might be some subtle difference in the shades of meaning between the two, but I'm not sure that's especially important here. If "easy" is what you want to say, go ahead and say it.
And yes, I think the sentence as a whole sounds natural.
One thing that might help you get your idea across a little better is if you added a bit of detail around "seemingly easy." A native speaker won't likely have much trouble inferring from your wording that you don't think English is actually all that easy, but without including some idea to set up the inclusion of "seemingly," it does sound slightly out of place to my ear.
You could make your idea more clear by saying something like
Every lesson helps me understand American culture and the nuances of its seemingly easy language.
That way, you're not saying that they help you understand the language as a whole while also randomly implying that you think it's not an easy language, but rather you're saying that they help you understand the parts of English that aren't all that easy.
The revision certainly isn't strictly necessary, but I think it makes your compliment sound even more complimentary.