In answer to your question. No, it's not rude. My mum came over on Xmas day ( we're currently in the UK ) and she called our family in Europe and had a very long loud phone call in another language. I couldn't understand a word but loved every minute. Any words she could teach my DD would be wonderful. Christmas is a time for sharing and family so the perfect forum.
A very British friend and her daughter were here and weren't interested and just occupied themselves doing something else. To hear the words was fascinating to me and we all had a laugh over mine and my husbands clear blunders of translation. I dunno. I find it all brings such richness to life.