今日の夕飯、何？って英語でなんて言うの？

'What.s on the menu this evening?' Obviously, at home there is no menu as in a restaurant, so this is a playful question, imagining your mother is a chef in a busy restauarant.

Dinner is a meal that is usually eaten in the evening.
One can also replace the word 'dinner' with 'supper' because the are synonyms of one another

Examples
A: What's for dinner?
B: Beef stew and dumplings

A: What's for supper?
B: Chicken noodle soup
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What are we eating tonight?

'tonight' can be used as dinner/supper is usually eaten in the evening.

Example
A: What are we eating tonight?
B: Chicken pie

What are we having for dinner?

A : What are we having for dinner?
B : I didn't cook so we can order pizza.
A : Sounds great!

What's for dinner?

C : What's for dinner?
D : We are having grilled fish and vegetables.
C : Okay, I am starving.

grilled - cook (food) using a grill
starving - very hungry

What's for dinner today?' - This is the most common way to ask what is for dinner.

'What's on the menu?' can be used in a playful way. I would often ask my mum this, knowing full well that there is no 'menu'. :)

Example:
Q: What's on the menu today?
A: We're having a roast dinner today. (A roast dinner consists of potatoes, meat and veg, usually all/party roasted.)

When we want to know what we are eating for dinner, we can use the following:

"What is for dinner, mom?"
"What's cooking, mom?" Informal
"What are we eating/having for dinner?"

"Mother, what are we having for dinner today?'' can be said when asking your mother what she is making for dinner.

What's for dinner?
What are we having for tea tonight?