Often, especially if you're relying on work experience rather than, or as well as, a degree. The Immigration Services Agency asks for Japanese translations of foreign-language documents, and your employer in Japan will tell you which letters it needs.
Under the Engineer/Specialist in Humanities/International Services status, relevant work experience can stand in for a degree: generally ten years, or three years for some international services roles such as translation or language teaching. That experience is proved with letters from past employers, which is where translations come in.
Intra-company transferees also need to show they've worked for the overseas company for at least a year before the transfer, and an employment letter is the usual evidence. Highly Skilled Professional applicants may claim points for experience and salary, which also need documents behind them.