This is an interesting localisation problem.
Obviously technical stuff: store everything in the database as UTF-8
and you are covered (well its not that simple but thats the basics).
That way the user can enter custom information for flooring types
that are japanese-only, or you can offer it as a choice in a drop down
Note that Globalise AFAIK assumes that you pick a dominant language
and stick to it. Translation is in effect unidirectional.
(Someone correct me on this if I am wrong). For the 'english reference'
I would just romanize it: 'tatami' or 'tatami (japanese floormats)'.
I am curious though, is tatami in this sense, just a flooring type?
In the same sense, that a user may have
wooden, hard-wood, carpeted, tiled, stone, linoleum, or PVC flooring etc.
Or is it a unit of measurement (like a 4-tatami room)?
I know that tatami have a uniform size and are as a consequence
used as a unit of room *size* measurement, in the same way that
we might otherwise use square metres or square feet.
If thats the case, I wouldn't try unit conversions between tatami