This is probably a minor issue, and a viewpoint that is not likely shared by anyone else, but I'll bring it up just to see...
I'm not used to seeing "let's" uncontracted; when it's written that way it feels like the speaker is actually asking permission from a third party, rather than speaking to his fellows and suggesting the salad. Does this make sense?
Yes I see what you mean it does look a little odd in an uncontracted form. I think the problem is that DL does not like contracted forms generally in English.
Once you think about the English phrase it is a bit odd in any case.
Alternatively "let's make a salad" sounds like a casual fun suggestion (if making a salad can be fun) while "let us make a salad" sounds like a group of monks taking the making of a salad very very seriously.
I would agree and wish for the sake of those wanting to use correct English that Duolingo update some of the English translations. It is always and everywhere "let's" It seems out of place to say "Let us". In French you would never say "Ce est" It's always "c'est" And there are contractions like that in English.
mphoenix12e is correct about the translation of your sentence. Be aware that it is a completely different thing, though. This lesson is about imperatives, which include the first-person plural form, "let's + verb", which is a way to encourage someone else to join with you in the activity. For this, the imperative is used in French. Your sentence is equivalent to asking permission (to be allowed) to make a salad.
Yes the pronunciation is quite different.
faisant rhymes with the French "an" (year) faisont rhymes with the French "ont" (as in ils ont une voiture).
But even if they sounded the same, you would know which is correct by thinking of the meaning. Just as you would know that you would not use "hair" in the sentence "The hare swerved and hopped and escaped the dog."