Learn Arabic (Modern Standard): الدجاج مشوي على الفحم.

It comes from the Form I defective verb شوى (to grill), which has a root ending in ya. Instead of the regular maf'uul pattern, defective verbs ending in ya merge the final letters into a ya with a shadda. This creates the pattern maf'iyy, giving us مشوي (mashwiyy).

Since الدجاج translates as chickens, shouldn't the adjective مشوي be plural or feminine?

The word دجاج is a collective noun, referring to chicken as a general category or meat rather than individual birds. In Arabic grammar, collective nouns are treated as masculine singular. That is why it takes the masculine singular adjective مشوي. If you were talking about one specific hen, you would use the feminine دجاجة.

Do I need to pronounce the double ya sound at the end of مشوي?

In strict Modern Standard Arabic, yes, you should pronounce the shadda so it sounds like mashwiyy. However, in casual reading and spoken dialects, people almost always soften or drop that heavy ending, making it sound just like mashwi.

Is على الفحم the standard way to say barbecued?

Yes! While it translates literally to on the charcoal, the phrase على الفحم is the standard, everyday expression you will see on restaurant menus to indicate that meat is charcoal-grilled or barbecued.