Translation:After he finishes eating, he will brush his teeth at once.
"After" is effectively implied in the Chinese here.
"When/once he finishes eating" is perhaps a more direct translation without "后", but the effective meaning is the same. Note that this sentence, like many Chinese sentences, can be categorized as a topic-comment sentence. Here the topic is "he finishes eating" and the comment is "(he) will immediately brush his teeth". There doesn't need to be an explicit link. The link is inherent in the way Chinese sentences work.
The sentence is normal without "后" – in spite of Google Translate – and the register is not markedly informal.
I think that for Duo the issue in EATandNAP's suggestion was the use of "meals" versus "eating", which seems like an okay alternative to me.
When I type 他吃完饭会马上刷牙 into Google Translate, it suggests an auto-correction: "Did you mean 他吃完饭后马上刷牙."
Google translates the current English phrase as: "吃完饭后，他会立即刷牙".
This exercise belongs to a skill about 后 and 以后. Since these phrases get edited over time, I was wondering if someone mistakenly replaced 后 at some point. I have no issue with EATandNAP's suggestion. My issue is that the implied "after" or "when" in the Chinese phrase seems odd.
他吃完饭会马上刷牙 = He finishes eating (he) will immediately brush his teeth.
There is nothing that links the first action to the second action.
Is this normal for Chinese or is it a type of informal speech that is used?
English also doesn't have future tense. We just have several future markers which have subtle distinctions: will vs shall vs going to vs gonna, plus you don't need to use any of them when talking about the future, as in "I go overseas tomorrow".
Chinese has some of the same subtleties of English and they each have unique subtleties.
I believe at least that 会 can mean both "will" and "would" for instance.
I agree that English doesn't have a future "tense", per se, and it's a helpful observation to make, particularly in this context.
It surprises people, though, because it's a nuance that's not typically taught in school. Indeed, traditionally the opposite has been taught, as well as thought, which means you'll probably find people disagreeing with you, though I suppose in the end "tense" and "aspect" are just descriptive tools to talk about "what we mean".
The English does not sound correct to me. I think it should be "after he has finished eating", future perfect tense. The whole sentence should be "He will immediately brush his teeth after he has finished eating" or "After he has finished eating, he will immediately brush his teeth".
Not to me. The Chinese sentence refers specifically to "brushing" the teeth, not "cleaning" them.
Also, we "brush" and "floss" our teeth (in North America, at least), but we usually go to the dentist to get them "cleaned". While it wouldn't be wrong, per se, to talk of cleaning them oneself (and there might be appropriate contexts), and while it may be regionally acceptable (though I haven't heard of it), it's just not what the Chinese says.
April 2020 the new correct way according to the APP is "after he finishes eating ,he will brush his teeth at once" thats soo wrong !! Mostly in chinese it will be said 他吃完了（后）马上刷牙。 He will brush his teeth right after finishing to eat。 The 会 in the sentence lets us know it will happen, so its not a situation that happened in the past. But actually in chinese it wil rarely be used, and we will know according to the context of the conversation