This is no problem at all. Yes they can take help from their group members for interpretation. Even if they are alone and require help, the border officials will have interpreters available for commonly spoken languages. Hindi is definitely one of them.
The U.K. does not require standard visitors to be fluent in English.
will they be sent back if they can’t speak in English?
No
Will my relatives face any issues at the time of getting visa from Embassy?
Usually there are no more visa interviews for UK for standard visitor visas. The staff on the application center will most probably be local from the country where an applicant files the application. So they can converse in their native language during application submission.
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