It would probably belong to a new class in the package org.apache.commons.lang3.text but I fear that nobody will ever find it there.
byteCountToDisplaySize() is a method that is strongly related to I/O - I can't think that we'd use that to print the size of String labels in a UI, for example.
So it should go into commons IO somewhere but maybe into IOUtils since it's not especially related to files.
And if you support I18n, you must allow to specify a locale. Otherwise, this code can't be used in a multi-language web app, for example.