Converting units into metric ones is quite a common translation practice in my country (especially when translating from languages of some backward countries). I wonder whether translators often do so translating into Japanese, or they just leave the original units and add footnotes to explain in metric terms.
There is no hard rule about this, and each translator decides whether they will convert units or not, taking many things into consideration.
- Young readers may not understand foreign units at all.
- The importance of each figure depends on the context. Most Japanese people cannot instantly tell if a 身長6.5フィートの男 is tall or not, but 身長2メートルの男 is instantly understandable as a tall man. If you need to convey the clear image of his height in the story, it's probably better to convert. But converting the length of Titanic (882 ft) may not be that important; usually all you need to understand is "it was a large ship" :)
- Nevertheless, unconverted and unfamiliar units can add a good, exotic atmosphere to the scene. Understandably, some people always dislike unit conversions. Probably no one wants to see 500 Miles translated as 800キロメートル.
For example, if I remember correctly, one dubbed version of Back To The Future uses 時速88マイル to describe the speed to achieve temporal displacement. Another version uses 時速142キロ, which is far familiar to the Japanese audience.