The quote is from the girls' lawyer. According to the article, the judge didn't actually say that was a mitigating circumstance.
Via the Daily Mail:
A gang of Muslim girls who repeatedly kicked a young woman in the head walked free from court after a judge heard they were 'not used to being drunk' because of their religion.
The group screamed 'kill the white slag' while kicks raining in on 22-year-old Rhea Page as she lay motionless on the ground, the court heard.
The attackers - three sisters and their cousin - were told by a judge that normally they would have been sent to jail.
However, he handed the girls - all Somalian Muslims - suspended sentences after hearing that they were not used to alcohol because their religion does not allow it.