Language Contact
Languages don't actually come into contact with each other. It is always the speakers of the languages who are in contact. Their attitudes towards each other will affect the way they speak. So, for those of us who study languages, it is convenient to simply talk about the languages as though they had a life of their own.
If speakers of a language want to identify with each other, they may find themselves adjusting their speech to eliminate the more obvious differences in pronunciation or vocabulary. If the speakers don't want to identify with each other, they may emphasize the differences in their speech, particularly if someone from the "outside" is present. Generally, the reasons why people want to associate with each other or not don't have much to do with the language they speak. It has more to do with the status or prestige of one or the other of the groups of people.
Speech differences come to represent social, political and geographic divisions between groups of people. These differences may also reflect differences in economic and political power. While we may react to someone's speech with a positive or negative attitude, we are really reacting to the whole complex of social, economic, and political connotations which we have associated with that speech variety.
When one group is very powerful they may use that power, unintentionally or otherwise, to attempt to eliminate the speakers of another language, or, as is more often the case, to eliminate their speech variety. Differences in economic or political power and prestige almost always put the (speakers of the) less-powerful language at a disadvantage. People may decide to stop speaking it in order to avoid the social stigma of being part of the less-powerful group. Sometimes, however, speakers of such a language resist having their identity (as marked by their language variety) taken away from them and they may react to the pressures to use the more prestigious language by working all the harder to preserve, protect and develop their traditional language of identity.
Wherever languages come in contact with each other there exists a greater need for at least some individuals to become bilingual. Bilinguals have proven very interesting to linguists who want to know how people cognitively and mentally organize the structure of their language(s) and how those structures may interact with each other. Bilingualism at the level of an entire society is of great interest to sociolinguists who want to know how communities determine "who speaks what language to whom, and when".