Considering the fact that alliances were sealed through marriages between nations in Europe, how often were these married couples unable to understand each other? Did they use translators to comminicate?
This is actually a really interesting question.
It's difficult to give you a completely authoritative answer but I can offer a few thoughts. First, it was often the case that royal children would know from a relatively young age who they would be marrying in the future when they came to maturity. This allowed them and their tutors to prepare them linguistically for their future partners, and in the cases of wives, the kinds of the cultural features they would encounter in their new home.
It was however common for medieval nobles to be bilingual or multilingual, and thus the children of the English aristocracy would generally know fluent French in addition to English, along with some degree of Latin. Susan E. Philips, "Chaucer's Language Lessons" The Chaucer Review 09/2011, Volume 46, Issue 1-2.
Thus, there was already a cultural impetus towards the acquisition of languages amongst the privileged classes across Europe, and these could be augmented with specialized language training in the event of an incompatibility.
Even amongst those who were lately contracted into marriage when they reached adulthood there would often be a significant period of time that would elapse as the details were worked out. These months, and even years in some cases, would afford the individuals in question the opportunity to acquire a familiarity with the language in question.