They could be Cambodian and Burmese also. I watch Burmese Satelite, they have similar culture as Laos and Thailand. Woman wears Sinh and man wears SALONG is the only difference.
Yes, I do agree that the ladies in Laos, Thai Xaing Mai, Burma and Combodia are look like to each other in terms of dressing, but in this photo they are surely Laotian. Why? The reason is obvious. The Tat Luang behind these ladies is symbol of Laos.