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The XLIFF 1.2 File Type is supported starting with WorldServer 11.1.1. So it has been around for quite a while now. However, it is not visible by default, you need to enable it as described in this RWS Community post and also add the file extension as Mime Type.
Not difficult at all if you follow the steps described in this article (click on the Preview and you will be redirected to the Knowledge article):
