"法兰绒衣服" can be translated as "flannel clothing" or more specifically, depending on the context, items like "flannel shirt"（法兰绒衬衫）, "flannel pajamas"（法兰绒睡衣） or "flannel jacket"（法兰绒夹克） etc.
If referring to clothes in general made of flannel fabric, "flannel clothing" is a suitable expression.