Can the employer in a manufacturing plant prohibit speaking another language on the production floor while working and implement a policy that English is the only spoken language in the plant? Can this policy be made that while the employee is on the company premises, the only spoken language is English?

Probably not but it depends on why you’re implementing the policy. Explanation: Requiring or banning a particular language to be spoken in the workplace is a form of ethnicity/nationality discrimination banned by human rights laws. However, otherwise discriminatory policies are justifiable if you can show that they’re a bona fide occupational requirement (BFOR). Example: Requiring a job applicant to speak unaccented English is discriminatory on its face. However, it might be OK for a public facing receptionist. More obvious example: Refusing to hire a job applicant because he/she’s visually impaired is disability discrimination unless the job is driver or other position for which sight is a BFOR.
Keep in mind that proving a BFOR is extremely difficult. To do it, you must show: 1. The policy serves a compelling, non-discriminatory purpose like safety? 2. You adopted the policy in the good faith belief that it was necessary to accomplish that purpose. And, most difficult of all, 3. That the policy really is necessary to accomplish that purpose and there are no less discriminatory alternatives available.
So look at your situation and determine whether you meet all 3 of the BFOR requirements, which is hard to do.  WHY are you requiring English only? Is there some compelling safety reason? Is the English-only policy necessary to achieve that safety objective? And are there alternatives that wouldn’t take away an employee’s language rights?
If you want to provide me more info about your situation, I’ll be happy to follow up and help you do the BFOR assessment. Glenn   glennd@bongarde.com

The only reason I’m asking this is because of constant complaints from fellow employees about one ethnic group who keeps on speaking their own language at the work station and during breaks.  They speak their own language while working with others who can’t speak or understand that language.  There have been many complaints about this from multiple people and the company talked to these individuals about the need to be courteous when others are around and avoid speaking a different language as others are getting offended and feel that you don’t want them to understand what you’re talking about especially when you can speak English.  This talk did not seem to be very effective as we still have complaints. How would you address an issue like this without being accused of discrimination?  Can the company discipline employees if they persist on speaking a different language when others are getting offended and continue to complain?

I’m afraid you don’t have any real legal recourse or grounds to discipline UNLESS the employees’ refusal to speak English with English-speaking co-workers is creating an intolerable safety risk.  Thus, for example, a Spanish-speaking supervisor in charge of confined space entry would have to speak English with crew members entering the space who don’t speak Spanish.
In extreme cases, I’ve actually seen lawsuits claiming that use of native language rather than English (or other prevailing language in the workplace) is nationality discrimination. See, https://hrinsider.ca/winners-losers-is-speaking-in-a-foreign-language-nationality-discrimination-against-english-speakers/  But I don’t think you want to go down the litigation route. I think the best solution is to get everybody in the room and work things out. The workers who are complaining should explain their concerns to their co-workers and to the extent the problem is creating a safety or productivity risk, you should seek to mediate a mutually acceptable solution. Example: You can speak any language you want EXCEPT when performing X,Y, and Z function which requires everybody to use the same language–English.
Hope that helps and pls. let me know what happens. I’m sure you’re not the only one having these kinds of issues and any lessons you can share would be much appreciated. Glenn