Should english become the language of the federal government of the united states of america. Yes it should and in pretty much already is.
Should english becoming the official language of the federal government effect a states choice in state recognized official languages. NO
Why because some states primary native population is not english speaking historically or currently i.e. : Texas(spanish,german)Hawaii (Japanese,hawaiian)California(chinese,spanish)Alaska(inuit,russian),Louisiana(french,spanish)Florida(spanish). Then there are community's like the native americans that have many different languages and I believe some amish community's speak dutch(netherlands).
And as far as immigrants learning english try and remember some of them have never had a formal education of any kind or very little education which means that they literally have problems learning period.