Translation:Mom, today we will have dinner at the restaurant.
Agreed. In the same way that compound future emphasizes process, so does English future continuous; they roughly correspond. While I'm glad they're not being so picky as to insist on it, continuous tenses should at least be accepted for imperfective verbs. Reported.
Not often. It's a very fancy and slightly old-fashioned sort of verb; you might say "tonight we will dine on lobster and caviar" to say, in a slightly joking manner, that you are having a high-class dinner. Perhaps British aristocrats and waiters in very high-end restaurants might say it, but the general public doesn't use it very often.
I think my intuition would have been to say «Сегодня мы поужинаем в ресторане». Does using the compound future add connotations? My train of thought would be that perhaps focusing on the process rather than the act emphasizes the shared nature of the activity. Is that about right?
I think in this sentence the process is important and not completion of an action. It's important that we will be eating, we will be sitting in the restaurant. I.e. the important part is how we will be eating (that we’ll spend more money and will have a first-class supper) and not the fact that we’ll actually be finish eating and be satiated afterwards.
But I'm not sure how that compares to the English sentence, and why the English sentence doesn’t use the continuous tense.
Of course, «Сего́дня мы поу́жинаем в рестора́не» works well too.
To hazard an answer to the question in the second paragraph, there are at least 8 locutions to express this future action. The short answer is that continuous tenses are more associated with informal usage, so in formal writing you would use more "shall" and "will." Beyond that, there are subtle differences.
Here's the cluttered answer. I shall use the more formal word "dine" (which was not accepted!)
Today we will dine in a restaurant--
Neutral or maybe a response acceding to a request or suggestion,
could be a decision just arrived at, or to distinguish from other days'
plans. If a specific time is mentioned--e.g. at eight o'clock-- it is the
beginning point of the action. Likewise in the next 4 examples.
Today we shall dine in a restaurant--
Neutral or an enthusiastic agreement to a request or speaker's own
Today we are dining in a restaurant-- Referring to a a plan already made and scheduled Today we dine in a restaurant Referring to a firmly scheduled event Today we are going to dine in a restaurant Spontaneous announcement, not a response to a suggestion, a future event planned by the speaker or others, not in response to a proposal by the addressee. Today we will be dining in a restaurant Referring to a plan already made, maybe in contrast to other occasions, but also savoring the anticipated experience itself. If there is specific time mentioned, it may mean the dining begin before and go on afterward (That goes for the following examples as well). Examples: "We will be dining at 8:00 pm today, in a restaurant." (referring to a scheduled occasion emphasizing the starting time.) "At 8:00 pm today we will be dining in a restaurant" (referring to the activity itself in progress on at that point, but we don't know when it started) Today we shall be dining in a restaurant Similar, but looking forward with even more pleasure Today we are going to be dining in a restaurant It's hard for me to put a finger on this, but I suppose it's like the foregoing but more informal. I would use it in a conversation like this: "Now, remember children, today we are going to be dining in a restaurant, so I expect you to behave yourselves."
This refers to a scheduled plan and the activity itself, which requires attention. "Will/shall be dining" also fit naturally here. It's a matter of style and taste.
Note that all this relates to the spoken language. In formal writing you see much less "going to" or continuous tenses in general.
Because сегодня means "today", you would need to say сегодня вечером if you mean "tonight.
Also, just for clarity, ужин is just the main meal of the day, and could be served either at lunch or in the evening. This is similar to the way that Thanksgiving Dinner, or Christmas Dinner can be served either in the middle of the day or in the evening.
On a side note, my wife comes from lower PA (US). They don't even have the word lunch :) Its breakfast, dinner, then supper. Confused the hell out of me for years. They would say we were having dinner at 1:00... just one of the many things my wife had to translate for me... and I grew up just 2 hrs south in MD.
Your mother has just told you that the stove/cooker is not working properly for some reason, and anyways she's sick and tired of cooking every day.... You can then spontaneously say, "Mom, we will have dinner at the restaurant today." Otherwise if you already have a plan, you would say "we're having" or "we're going to have". Surprisingly, for non-native speakers, we only use "will" for the future in certain specific cases.
I agree with your scenario for using 'will'.
Whether or not the original Russian sentence is appropriate in that context, I can't say. My Russian isn't good enough for that yet (and probably never will be, at this rate).
My gripe with Duo regarding the sentence in question is that it accepts - even gives as its preferred answer, "Mom, today we will have dinner at the restaurant", but will not accept what amounts to the sentence if you move 'will' to the end. As far as I am concerned, the latter is as correct as the former!