Apologies for the title; I don't mean to set up the 30 years war as the first case or anything; it's simply the earliest I can actually picture there being fraternization.
A reply to /u/MamaSexyEggs
Can you more precisely define what is "fraternizing" and what is "different European armies", please? In that era there was not yet the concept of nation-states that we have today, so the short answer is that your question does not apply very well to that era.
Beyond this, all I can say is that there was significant use of mercenaries in that era, and the late medieval era, such that it was very common for soldiers to change employment, for units to change sides, and thus another short answer is, "Of course, because there were no nation-states in the first place."