For most of the NGPC games, they will play in English if you set your system to English. Later on, SNK made games more territory-specific, like with Last Blade (J) playing in Japanese no matter what.
Still, I think the majority of titles still have both Japanese and English in them (especially the older ones). My copies of KOF R-1, KOF R-2, Samurai Shodown 2, and Metal Slug: 1st Mission all have dual-language support.