How often do husbands and wives don't speak the same language? Would one of them try to learn the other's language before the marriage? Or do they have translators?

You also need to consider that in certain scenarios (e.g. a king might only have daughters), marrying an heir of to a different throne could mean securing that throne for your dynasty some time in the future.

The Habsburgs were especially good at this, hence the motto "Bella gerant alii, tu felix Austria nube" ("Others may fight wars, but you, happy Austria, marry!") Emperor Maximilian secured Burgundy via marriage (1477, although Maximilian and Maria of Burgundy apparently acutally loved each other, which was uncommon), and then organised an entire congress in order to marry his grandchildren into the Jagellonian Dynasty, which later secured Hungary and Bohemia for the Habsburgs.

Just a further question, if that's allowed: a marriage would involve the church, would that have made the 'contract' more binding than just an agreement between leaders?

I think another thing to remember is that wars were costly. In England for example, there was no standing army and the King would have to raise taxes and men from his noble families in order to go to war. It could cripple the kingdom. A good example of a King who used marriages to cement security and financial stability was Henry VII. He made auspicious marriages for his children in order to shore up alliances - His eldest son Arthur to Catherine of Aragon to secure the Spanish alliance. When Arthur died it was such an important alliance to keep that Henry then married his second son, the future Henry VIII to Catherine. It was, if nothing else, a way of keeping certain countries off your back to allow you to focus your energies on the key enemies. If you didn't ally then your enemy might make an alliance - for England that could be dangerous, being an island. Likewise for France in medieval times and into the early modern period, they were becoming encircled by the Hapsburgs.

I suppose the other thing to remember is that marriage is a sacrament of the Catholic church. In that respect it was seen as an authoritative bond that people were wary to break. Look at what Henry VIII had to do to get out of his marriage, separate from the church in Rome! A legitimate marriage, made and consummated in the eyes of God, was a binding contract. People weren't willing to risk it which is why you see examples of marriages trying to be broken on questions of consummation and legitimacy rather than risk just throwing over the marriage for foreign policy aims.

Difficult question - historians are still debating part one of your question. It is important to note that both sides/families were believed to benefit from these marriages not just the family of the groom.

The benefits to the groom's family are probably more obvious - a woman, who will in time become a mother, which will continue the family line and secure the succession to the throne. In the future the children of this marriage would have the benefit of their maternal relatives to call upon for support and alliances. The groom's family had the benefit of the dowry and could expect to gain either power, influence, money or an alliance from the family of the bride.

The bride's family on the other hand also gained. Her relatives could expect an increase in influence or position for possibly two generations (see for the example the rise of families such as the Woodvilles, Boleyns, Seymours and Howards in England). The wife also had access to her husband, sounds obvious but this could be very important, she could provide information to her family which they could use to their benefit, as well as promoting her own relatives whenever possible. Many queens were able to arrange influential marriages for their sisters which perpetuated the upward cycle of the family. Finally wives were provided with a financial settlement (like a pension) in case of the early death of husband, she could also expect to have influence over her children (possibly a regency or joint-regency), and some wives effectively ruled in their husband's place if they were incapacitated, on crusade or at war (see particularly Maria of Castile, wife of Alfonso V of Aragon. Alfonso was absent from his kingdom for over 25 years, during which time Maria ruled in his place)

I've been reading the replies here, and while many have brought up many interesting points, I haven't seen anyone reference belief systems yet. In medieval Europe, most people practiced Catholicism and were highly religious. Depending on how religious the country you were in was, just the act of promising to marry someone could legally be considered a marriage. Once you married someone, the only way to receive an annulment was by petitioning the Catholic church and they were generally only allowed if the marriage had not been consummated. In a woman's case, if the marriage was annulled, she would lose anything she'd gained from the marriage, including property and status, and would possibly be stigmatized by society depending on the reason for the separation. She would revert to her parents' care and be completely reliant on them and other family for financial support. The consideration of an annulment was not something a woman especially would take lightly. If you were a man, the repercussions may not have been as weighty, as any failure in the marriage would most likely be blamed on the female in the relationship.

Marriage was a highly sanctified and revered holy right to medieval European people. As previously stated, marriage was not something one could get out of easily, so they were more strategically planned than today. People were not married on a whim or for love very often; matches were painstakingly arranged and negotiated the way a business merger might be today. Marriage was a way to create alliances and gain power, so breaking one wouldn't be very beneficial to the couple in most circumstances. The marriage contract was both spiritually and legally binding, so there would be financial, religious, and political repercussions on both sides if the contract was broken. There were exceptions, of course, but this is a large part of why most people did not break marriage alliances.

I'll list some notable examples of why people did or did not break marriage alliances as my reference. I've posted about Henry VIII elsewhere in this thread, and I'll use he and his father as examples here as well because their marriages are excellent examples of how matrimony could affect both religion and politics in late medieval/early Renaissance Europe.

• Henry Tudor, as he was known before becoming king, did not have a particularly legitimate claim to the throne of England. He legitimized himself through rigorous political propaganda and his marriage to Elizabeth of York. Elizabeth was Richard III's niece and was, as many people believed, the rightful heir to the English crown as the eldest living child of Edward IV. Henry Tudor was able to win support for his invasion of England based upon an alliance with the Yorks to marry Elizabeth and make her his queen. Upon his coronation, Henry VII did not immediately marry Elizabeth, and this almost had dire political consequences for him. Henry specifically did not marry Elizabeth because he was asserting his right to the throne regardless of his marital status. He wanted power in his own right, not to be a puppet of his wife's family's power and influence, so he stood his ground. Ultimately, his plan worked, and he did end up marrying Elizabeth. Their marriage, and the alliance of the Tudors and Yorks, ended the War of the Roses and created the Tudor dynasty. This is an example of why a king would honor a marriage alliance even if he didn't particularly want to for whatever reason. (Source: Winter King: Henry VII and the Dawn of Tudor England)

• Henry VIII famously "divorced" his wife, Catherine of Aragon, so he could wed Anne Boleyn. The attempted annulment of their marriage destroyed the long alliance England had with Spain, as Catherine was the daughter of Queen Isabella and King Ferdinand. Henry's marriage to Catherine was part of the alliance contract between the two countries. At the time, Henry was interested in a new alliance with France instead, and one of their most hated enemies was Spain. Henry's newly intended wife, Anne Boleyn, had spent years living in France and was in favor of an alliance between the two countries. You can see how, in this case, having the marriage annulled and breaking the alliance would benefit Henry in several ways. Historians can argue about the pros and cons of the marriage alliance being broken, but this single act pushed forward the Protestant Reformation and had political repercussions through ought Europe for generations. (Source: The Wives of Henry VIII)

• Catherine of Aragon's sister, Joanna of Castile, had her own marriage alliance woes. She was married off to Philip, Duke of Burgundy, who was the son of Maximilian I, Holy Roman Emperor. Their marriage was part of an alliance with the Habsburgs intended to strengthen them both against growing French power. Unfortunately, while at first the marriage was a spectacular success, the death of Queen Isabella created a vacuum in power which threatened the political stability of the alliance. Though the throne of Castile technically should have been Joanna's by right, her husband and father proceeded to fight over which one of them would rule it. King Ferdinand wanted to rule in Joanna's name, as she was female and rumored to be insane and mentally incapable of ruling. Phillip disagreed and wanted power instead, so he decided to travel there and rule it by force in his wife's name. As if this weren't bad enough, after a short honeymoon phase, Philip began courting other women, and Joanna became insanely jealous. The alliance between the two families became strained, but Ferdinand lacked the resources to challenge Phillip's rule of Castile. Phillip died of typhoid fever in mysterious circumstances soon after; many contemporaries and historians have speculated that Ferdinand had Phillip poisoned so he could retain power of Castile in Joanna's name. Joanna attempted unsuccessfully to rule Castile on her own and ended her life reportedly insane and locked away in a covenant by her own son.

I hope that answers your question satisfactorily. Sorry for the longevity... I like to rant. ;)