From my certificates, you can see I have a reasonable knowledge of Italian and Spanish.

reasonable 是形容词，由 reason（理由、理性）加后缀 -able（能够...的）构成，原意是"合乎情理的"。在这里引申为"相当程度的、适度的"，表示知识水平达到了一定标准但并非精通。