To say that any particular word in any particular language is the most beautiful word in that language presupposes a standard of beauty and some definition of word. Is it the sound of the word? The look of the written word? If it's the concept behind the word it's not really the word per se that's beautiful and I would submit that this is the case with mother. If this is the case, then it doesn't really make sense to say that it's the word that's beautiful nor that it's particularly beautiful in that language as opposed to other languages. We also have the fact that beauty is a very subjective quality. With all these variables I can't see any reason to disagree with nuncle's statement.