Neither pant nor trouser are english words for a piece of clothing you can put on your legs. Both only come in the plural- trousers and pants as abbreviation for a pair of..... A pant is a short or briefly shock of breath and the word trouser in sg. only exist as a hyphon combination, t.i.t.s. trouser-clip, trouser-zip a.s.o.....
Because 'suruali' is in class 9/10 (N/N), which uses 'yangu' for singular and 'zangu' for plural. (As niels862850 explained, in standard English this item of clothing is considered plural but in Swahili it is singular.)
'Langu' is for class 5 nouns, e.g. 'shati langu' (my shirt).
If you mean the word for clothes/clothing, "nguo" is in class 9/10. But particular items/types of clothing don't belong to one class. For example, "shati" (shirt), "gauni" (dress) and "koti" (coat, jacket) belong to class 5/6, whereas "suruali" (trousers/pants), "sketi" (skirt) and "soksi" (socks) belong to class 9/10. I may be wrong, but I think most clothes are in these two classes. However, "kiatu" (shoe) belongs to class 7/8.
So the only way to be sure is to look up the word in a dictionary (and even they can disagree). Try this one: https://africanlanguages.com/swahili/
Suruali comes ultimately from Persian شلوار (šalvâr, "trousers; shalwar"), from Proto-Indo-European *skelo- (“thigh”, root of Greek σκέλος [skélos, "leg"], German schielen ["to squint; to leer"] and perhaps of Spanish calzar ["to wear or put on shoes"]) + *wero- (“to cover”, root of numerous English words like ware, aware, weir, ward, lord, warn, garrison, guarantee, warranty and garnish).