I don t think there are any specific rules on this. But there are rules on things like listing ingredients, providing warranties, giving safety warnings, size and washing instructions etc. These are usually provided on or in the box and clearly, for the UK market they will need to be in English.Obviously the rules that apply depend on the type of product so it is impossible to give accurate advice here.You also need to be aware of other rules that may apply to certain imported products, not just related to language. Germany is part of the EU common market, so there will not be many restrictions. But there are some rules. EG it is illegal to sell electrical goods without a properly fitted UK specification plug.