I have always taken the view that people employed should be able to read, write and speak English to a sufficient level to be able to do their job and understand important instructions (especially health and safety for example).
I've had a look on the equality website and this is what I found:
http://www.equalityhumanrights.com/uploaded_files/code_of_practice_on_racial_equality_in_employment.pdf
"There is a clear business interest in having a common language in the workplace, to avoid misunderstandings, with all the risks these can entail, whether legal, financial or in relation to health and safety. It is also a matter of courtesy, conducive to good working relations, not to exclude people from conversations that might concern them, when they are present. In the main, English is the language of business in Britain and is likely to be the preferred language of communication in most workplaces (see Example 15 on p 57), unless other languages are specifically required."
"Where the workforce includes people who are not proficient in the language of the workplace, employers should consider taking reasonable steps to improve communication (see Example I at para 4.58). These might include providing:
a. interpreting and translation facilities; for example, multilingual safety signs and notices, to make sure workers understand health and safety requirements;
b. training in language and communication skills; and
c. training for managers and supervisors on the various populations and cultures that make up Britain today."
So it appears that you can do multilingual signs although there is no mention about it having to be in all languages of the site; however, if you have one Russian worker for example and your signs are in Polish and English, I can see how that could be discriminatory. They do give another option above though and that is to train people. I would work with your union (if you have one) to provide English basic skills courses and as Anne said put as much information visually as possible to avoid language skills being an issue and train your management in doing this too. I think interpreting all your signs is a slippery path. Where do you stop? It will slow down your pace of change considerably and may worsen any "us and them" atmosphere which is already brewing.