Not just in Polish - word Niemiec, Nemec, немец etc. you will find used (not necessarily as the most common word nowadays) in many Slavian languages. Simply be

Not just in Polish - word Niemiec, Nemec, немец etc. you will find used (not necessarily as the most common word nowadays) in many Slavian languages. Simply because Slavians share common roots of the language and can more or less communicate with each other while Germanic tribes were 'non-speakers'.