Working Retail Without Speaking English?

Where I live English is REQUIRED.

Depends where they need help and how much they need help.  She'll pick up English pretty quick at least enough to start to understand what needs to be done in say a stockroom.  It's unlikely to say the least that they would put her on a cash register or customer service.

There are some parts of the country that speak spanish predominately. It depends on the area. 

How is she going to be hired if she can't communicate with her co-workers, or boss? She absolutely needs to learn SOME English to get a job, especially if she has to deal with the public, such as you do in retail. 

She can't take a translator along with her to work. She's going to have to be able to understand the forms she needs to sign, the implications of what she PUTS on those forms and she will need to be able to talk to supervisors and co-workers all the time. I'd say her chances of being hired are really slim until she can. 

Not good.  First of all, your sister-in-law actually needs a work visa to be legally employed anywhere.  Second, if she doesn't understand or speak basically English,  she'd have a very difficult time understanding instructions or being able to ask questions of her supervisor.

not going to happen

Most customers in the US and Canada use English, if you don't speak it you may have issues with doing your job properly and helping customers. You may be able to help her out (if you work on the same shifts). Most work places require you to be able to understand English, meaning being able to read, speak and write in English. She should take some ESL courses before she gets a job. 

I'm not sure about that.  Under typical circumstances, I would suggest that your SIL consider ESL (English as a Second Language) tutoring, such as through the local public library.  There may be a free tutoring program, such as though the Literacy Volunteers of America, which may be offered through the local public library.

Communication skills are important in most jobs.  How can she do her job if they cannot communicate her tasks to her?

I would imagine chances are not good. But if they have a "back room" position available, she speaks a language common in the area, and the store is desperate for employees, it's possible.