Did nationalism -- or proto-nationalist sentiment -- exist in late-medieval and early-modern Europe?
No, says Daniel Nexon:
"Early modern European politics...lacked a fundamental feature of contemporary politics: nationalism. The tight connection between nation and the state represented by the ideal of 'national self-determination' simply did not exist in dynastic agglomerations.... Local identities remained far more important than the still inchoate notions of patria or nation.... Dynasts might, often through propaganda, argue that their dynastic interests and the interests of their holdings were synonymous. Sometimes they succeeded, but there was nothing obvious about the harmony of dynastic ambitions and the interests of a kingdom or principality. As J.H. Shennan remarks, 'We should beware of misinterpreting aspects of the consolidation of princely authority in the late fifteenth and early sixteenth centuries as evidence of proto-national sentiment.' " 
Yes, says Norman Housley:
"...the extent to which the members of Europe's political communities felt united by national feeling in the late Middle Ages remains hotly disputed.... Bernard Guenée provided a useful analytical framework for the ways in which national sentiment developed. Nomenclature (Francia, Alemania, Polonia, and so on), a common language, a sense of geographical cohesion, shared religious traditions (above all, 'national' saints), and the consciousness of a common history, were all important motors for national feeling; while for the historian, they form indices for the strength of that feeling at any given point. Thus, for Guenée, 'the fact that neither the states nor the subjects of the duke of Burgundy had a common name was more of a threat to Charles the Bold than the policies of Louis XI'.... Guenée and others have also placed emphasis on dynastic continuity, administrative advances, a sense of resentment against privileged and intrusive foreigners, and above all the homogenizing burden of war, as contributory factors in the growth of national feelings. Some historians, including myself, believe that the result was a national sentiment of considerable vigour, which was closely bound up with the development of European statehood." -----------------------
1. Daniel H. Nexon, The Struggle for Power in Early Modern Europe (Princeton Univ. Press, 2009), pp. 94-95.
2. Norman Housley, "Pro deo et patria mori: Sanctified Patriotism in Europe, 1400-1600," in P. Contamine, ed., War and Competition Between States (Clarendon Press/Oxford Univ. Press, 2000), pp. 225-226.