The way I have done is via CPanel. I log into my CPanel account (www.yourwebsitename.com/cpanel) >> public folder >> app >> locale >> translations. Then, you shall create a folder for the desired translation; something like jp_JP. Create/upload a .csv file with the original languade between " " followed by a ; symbol and the word in Japanese, between " ".
From this way, the translations will run accordingly if you change the languages.
Best regards.