Writing Sample Topics: Applying for History Phd
In my limited, but certainly present*, experience with history departments, your writing sample wont be used as a negative indication of language abilities. Your statement of interest and and language test/coursework proof will be noted, but if you have no Portuguese in your sample they are unlikely to notice. Your interests sound like a natural progression and they do not expect to see undergraduate work that directly relates to your proposed topic. If you're worried about it, sit a formal Portuguese test of some kind and report that on your cv. 

*before grad school I was the secretary of a history department in the usa and managed admissions admin for it.