The word block can refer to
A lump of wood, stone, or other matter, that obstructs one's way.
(Oxford English Dictionary (OED)).
This refers to a literal lump of wood or stone that you can pick up or remove. According to the OED this usage of block now occurs only in the compound noun stumbling block.
For example, you could say
Watch out for those bricks in the road ahead. They might cause you to stumble or trip. They can be stumbling blocks for anyone who does not walk carefully here.
All this refers to actual physical bricks, which form actual physical stumbling blocks to people who might literally stumble over them. You can actually pick up the bricks (which form the literal stumbling blocks) and clear the path.
The word stumbling block does not normally refer to a physical block or stone.
The link above (Oxford Dictionary online) lists several example sentences. An example off the top of my head is
The growing cost of higher education forms a stumbling block to parents who wish to send several children to university.
Here stumbling block is used figuratively. And that is how it is usually used.
Common synonyms for stumbling block include obstacle and hurdle.
To insist that a stumbling block must be something we can see is wrong.
However, it can refer to something physical:
The pimple on my sweetheart's nose is a stumbling block to me asking her to get married.
but here it's the existence of the pimple that forms a stumbling block or obstacle in the mind of the speaker. It is a physical object that is forming a mental stumbling block to the speaker.
I am not sure what your professor means by
A stumbling block holds back water in a reservoir.
It seems that he is using or trying to use stumbling block in the literal sense, to refer to a literal block or lump of wood or stone or similar that impedes or obstructs the water's progress, so that a reservoir is formed. But stumbling block causes people to stumble, not water or other "inanimate objects."
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.
Best Answer
Please use English, Please use the English language and In English please are all equivalent in meaning.
Depending on usage, English implies the language, not the people, so, Please use English would suffice. The English are the people:
Please use the English language is correct, but bulky, as if spoken by or spoken to a non-native English speaker.