The average Pennsylvania resident would probably instinctually answer "no" to the question of whether an employer can discriminate against applicants or employees based on the language that they speak. However, the answer to this question is fairly complex and depends largely on the nature of the discrimination and whether or not it is tied to one of the categories of protection offered by current employment law.
Employers are prohibited from discriminating based on national origin, but in the area of language it is difficult to know how far that extends, especially for employees who were born in the United States but happen to be multilingual or have a first language other than English.
One major issue is whether an employee is being treated differently based on the language that they speak which gives the appearance that they are from another country. Obviously, it is unacceptable for employers to discriminate for any reason, but the law protects employees from discrimination and harassment based on only a few selected categories.
The situation usually has to be relatively extreme to rise to the level of harassment, generally it has to compel the allegedly harassed person to quit their job. Most people think about harassment concerns being raised by the person who is in the minority, but in this situation it could just as easily come from a native English speaker who is concerned about the content or nature of conversations being had in other languages at work.
It is clear that the increasing numbers of international workers in the United States as well as employees who simply come from multilingual families, these issues are bound to become more and more prominent in workplaces in the coming years. Since 1980, the number of people in the United States who speak a language other than English in the home has increased by 140 percent.
Source: Pittsburgh Post-Gazette, "Rise in multilingual employees increases 'English only' discrimination issues nationwide in the workplace," Erich Schwartzel, Sept. 10, 2012.