Apparently it is technically difficult to do so implementation of the idea has been deferred, see Preferences Language.
From a general practical perspective, dual language customisation is going to be difficult due to
- Double the space is required for all titles
- Customisation once for each language is a significant task. Their are far more pairs of languages which could be simultaneously shown.
- Program performance implementation identified earlier
Other programs such as GnuCash do attempt to support this type of functionality with a language specifiable for user, supplier and customer. Clearly a major feature to embark on, as such looking for a work around or ways of working with what is available is likely to be more rewarding.