"Nous sommes en train d'acheter des fruits."
Translation:We are buying fruit.
"être en train de + infinitive" means "to be in the process of, in the middle of". It expresses something that is happening right now. Like
Je suis en train de dîner → I am (in the middle of) having dinner.
In this sentence it means "We are (in the process of) buying fruit."
...in the process of... must be right here - at least as an option. You need to express the immediacy of the action, and "We are buying fruit" does not do that unambiguously. Nous achetons des fruits could also be translated as "We are buying fruit" but it does not mean the same thing.
we are buying fruits did not make it, and it is not idiomatic, but it it grammatical and not a semantic violation. But my question is, is there a fruit/fruits issue in French as well, since des fruits is plural, no? My Practique Larousse give fruit as masculine, so what would I be saying in French if I used du fruit?
des is the plural of un / une.
un fruit is a single piece of fruit. des fruits is fruit plural. In English we use the word fruit as a plural and, to singularise it, we either say "a piece of fruit" or, for example, "a tomato is a fruit".
I would say, if it were a single piece of fruit being purchased, then the French sentence would be Nous sommes entrain d'acheter un fruit.
I don't know if that helps at all. :-)