"Let us play soccer."
Translation:Jouons au football.
I understand the translation, but I dont think any British English speaker has said "Let us do such and such" since about 1910! No idea about other parts of the world. We would say "let's". Pretty much the only exceptions are very formal instructions (rather than invititation-suggestions) such as "Let us pray" or "Let us take a moment to rembember those who died".
We can't enter "let's" in the main sentence for translation to French because the system does not like apostrophes. But of course "let's play" is accepted when you get the French sentence for translation to English.