"Drink clean and safe water"
Translation:Kunyweni maji safi na salama
To avoid using (sing.) and (pl.) in the English sentences, and to avoid so-called southern English -- "Y'all drink clean water!", the command could be preceded by a plural word, e.g., "Students, drink clean and safe water!" Then it is clear the plural imperative is needed in this sentence.
Edit: In the latest revision of the Duo French course, they are prefacing the command (or whatever) with the names of two people to make it clear that a plural form is needed, e.g., Rashidi and Juma, go to the head teacher's office ~ Rashidi na Juma, nendeni ofisi ya mwalimu mkuu.