They did not share a common spoken tongue. But the Portuguese had been trading in China since 1515, and the Japanese had been trading with China for much longer; they communicated with written Chinese. Additionally, both Japanese and Portuguese accounts agree that a Chinese man was among the group that originally landed with the first Portuguese to set foot on Japan, and that someone in the town they landed in knew Chinese.

Source: Tanegashima: The Arrival of Europe in Japan by O. G. Lidin.