Not only is “sukoshi” Japanese, it is extreeemely basic Japanese.
W. E. Griffis, _The Mikado’s Empire_ (1913), p. 363:
//Every foreigner who sojourns in Japan for a week learns “Sukoshi matte” (wait a little), “Ikura?” (how much?), “Doko?” (where?), ....//
A. M. Z. Norman, “Bamboo English”, _American Speech_ 30[1]:44 (1955):
//Bamboo English [English of GI’s in Japan] employs _sukoshi_ ‘few, some’ and its antonym _takusan_ ‘plenty,’ both of which are forthwith made into two-syllable words, dispensing with the voiceless Japanese _u_.//
Norman goes on to predict that most of the adopted Japanese words—including “sukoshi”—will fail to persist in English. But “sukoshi” = “skosh” has made it after all, I guess.