Nia mendo estas simpla: unu kliento volas supon, alia volas rizon, kaj ĉiuj ricevas po unu glason da akvo.

Yes, but mendo can mean the whole order as one combined order.

So Nia mendo estas simpla treats everything the group is ordering as a single order. English does the same: Our order is simple.

If you were talking about several separate orders, you could say niaj mendoj.

Because simpla is a predicate adjective after estas.

In Esperanto, an adjective after esti describes the subject; it is not a direct object, so it does not take the accusative -n.

• La mendo estas simpla. = The order is simple.
• Mi vidas simplan mendon. = I see a simple order.

In the second sentence, simplan mendon is the direct object, so it gets -n.

Because they are the direct objects of volas.

• volas supon = wants soup
• volas rizon = wants rice

The accusative -n marks what is being wanted. This is true even with mass nouns like supo and rizo.

It could be alia kliento, but Esperanto often leaves out a repeated noun when it is obvious from context.

So:

• unu kliento volas supon, alia volas rizon

means:

• one customer wants soup, another customer wants rice

This works much like English one customer..., another...

Usually alia just means another or a different one.

In English, if there are exactly two people, we often say the other. Esperanto may still use just alia in that situation, especially when the meaning is clear from context.

If you want to make the idea of the other one more explicit, you can say la alia.

Ĉiuj means all of them / everyone as a plural group.

• ĉiuj = all, everybody
• ĉiu = each one, every individual one

In this sentence, ĉiuj ricevas po unu glason da akvo presents the customers as a group, so ĉiuj is very natural.

You could also say ĉiu ricevas po unu glason da akvo if you want to emphasize each individual separately. Both are possible, but the nuance is slightly different.

Po is a distributive word. It means something like each, apiece, or per person.

So:

• ĉiuj ricevas po unu glason da akvo

means that each person receives one glass of water.

Without po, the sentence would be less explicitly distributive. Po makes it completely clear that the amount is assigned separately to each person.

You may also notice the form glason. Many speakers use the accusative here because the whole phrase functions as the object of ricevas. You may also come across po unu glaso da akvo in other texts.

Da is used after words showing amount, measure, or container:

• glaso da akvo = a glass of water
• kilo da rizo = a kilo of rice
• multe da homoj = many people

So glason da akvo is the normal Esperanto pattern.

De usually has other meanings, such as possession, source, or relation, so it is not the usual choice here.

Also, after da, the following noun normally stays in its basic form:

• da akvo, not da akvon

That is because the main noun here is glason. The direct object is the glass, while akvo is part of the da-phrase describing what the glass contains.