European countries (including France, Austria and the UK) all require that children be restrained by a car seat that meets European regulation R129 or R144. Most US car seats are not tested to this European standard but instead to a US standard so are technically not legal in Europe. That does not mean that they won't keep your child safe just that you are in breach of the law. The same would apply if a European car seat was to be used in the US as they are not tested to meet US standards.