static areSiblings(a, b) {
    const LanguageUtils = shaka.util.LanguageUtils;

    const baseA = LanguageUtils.getBase(a);
    const baseB = LanguageUtils.getBase(b);

    return a != baseA && b != baseB && baseA == baseB;
  }
Check if two language codes are siblings. Language codes are siblings if
they share the same base language while neither one is the base language.

For example, "en-US" and "en-CA" are siblings but "en-US" and "en" are not
siblings.

@param {string} a
@param {string} b
@return {boolean}
@export