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Yes there is. WP:ENG: ""Citations to non-English sources are allowed. However, because this is the English-language Wikipedia, English-language sources are preferred over non-English ones, whenever English sources of equal quality and relevance are available"". For general claims like that which has plenty of English language sources, English language sources are preferred. There is simply no need to use a Polish source for general statements about Fredericks rule. Polish sources are only preferable for subjects that lack English language sources, like perhaps specifics regarding Poland during his time.   "