In the movie's Wikipedia article, there is a French poster of the movie, which says "Le Discours d'un Roi". This is interesting to me because the French title uses the indefinite article "un" to refer to the king, which in my view makes more sense because he's just a king and from the title alone it's not obvious which king we are talking about.
Is it more appropriate that the English title use the definite article? Is it because the king is British?