When entering any country the general expectation is that you will follow their rules and regulations. What isn’t always practiced in the United States is that you have a basic understanding of the English language. Thanks to interpreters and the high demand for their services learning English isn’t always the highest priority for immigrants. Without sounding too demanding, a basic understanding of the language is desired by most individuals that are either American-born or have been in the country long enough to learn this custom is at times considered a necessity. Many other countries feel quite the same, and will offer interpretive services, but when dealing with immigration will still insist that the person applying for residency show some proficiency with the native language.
In many situations this is considered common courtesy, as immigrants are very welcome in the USA and are encouraged to seek their fortune and success when relocating. In order to remain culturally diverse and not begin to divide people into their native groups as a rule it is sometimes necessary to learn the common language of the land just to get by. This can make things a great deal easier on everyone involved and can help facilitate an easier adjustment period when acclimating to the states.
Which language is most dominant is very dependent on what region of the USA an immigrant travels to. For instance, in parts of California, Texas, and even Oregon, Spanish is a widely accepted language, though English is still the dominant tongue. If you travel to Louisiana you might hear a good deal of French, and in Florida you might hear more Spanish. Wherever you go in America, English is bound to be the dominant language, and it would be good to brush up on before arriving, if only to insure that there are no pesky language barriers that can hamper any chance of success.
One thing to remember about English however is that it can be a very confusing language. Unlike the rest of the world English is a language in which one word or phrase can have multiple meanings. This can often confuse native Americans, which means that immigrants that are able to speak only passable English might be lost completely during a conversation. The best advice I could think of to give anyone thinking of immigrating to America is this: if you want to learn English, watch English television shows. It might sound odd, but you will learn more from TV when it comes to speaking conversational English than any school could teach you.