Aromantic in Spanish is "arromántico" with double R because, between vowels, it matters for the pronunciation if you have single or double R. Notice that in Spanish adjectives take desinences for gender and number: "un chico arromántico" (an aromantic boy) vs. "una chica arromántica" (an aromantic girl). Anyway, it can be shortened as "arro", which is invariant, though it may admit a plural as a noun ("los arros").