function selectLanguage(options, lang) {
    let langToUse = lang;
    if (langToUse === '' && options.defaultLanguageForUnspecified !== undefined) {
        langToUse = options.defaultLanguageForUnspecified;
    }
    let prismLang = loadPrismLang(langToUse);
    if (prismLang === undefined && options.defaultLanguageForUnknown !== undefined) {
        langToUse = options.defaultLanguageForUnknown;
        prismLang = loadPrismLang(langToUse);
    }
    return [langToUse, prismLang];
}
Select the language to use for highlighting, based on the provided options and the specified language.

@param {Object} options
       The options that were used to initialise the plugin.
@param {String} lang
       Code of the language to highlight the text in.
@return {Array} An array where the first element is the name of the language to use, and the second element is the PRISM language object for that language.