Prefer lang global directive than constructor option

src/markdown/directives/apply.js

@@ -70,8 +70,8 @@ function _apply(md, opts = {}) {
           }
 
           // Apply attribute to token
-          if (lang || marpitDirectives.lang)
-            token.attrSet('lang', lang || marpitDirectives.lang)
+          if (marpitDirectives.lang || lang)
+            token.attrSet('lang', marpitDirectives.lang || lang)
 
           if (marpitDirectives.class)
             token.attrJoin('class', marpitDirectives.class)

src/marpit.js

@@ -72,6 +72,8 @@ class Marpit {
    *     slide page at before of headings. it would apply to headings whose
    *     larger than or equal to the specified level if a number is given, or
    *     ONLY specified levels if a number array.
+   * @param {string} [opts.lang] Set the default `lang` attribute of each slide.
+   *     It can override by `lang` global directive in the Markdown.
    * @param {boolean} [opts.looseYAML=false] Allow loose YAML parsing in
    *     built-in directives, and custom directives defined in current instance.
    * @param {MarkdownIt|string|Object|Array} [opts.markdown] An instance of

test/markdown/directives/apply.js

@@ -120,6 +120,11 @@ describe('Marpit directives apply plugin', () => {
         expect(sections.eq(1).attr('lang')).toBe('en-US')
       })
 
+      it('can override the language that is setting by `lang` constructor option', () => {
+        const $lang = load(md({ options: { lang: 'fr' } }).render(langDir))
+        expect($lang('section').first().attr('lang')).toBe('en-US')
+      })
+
       context('when lang directive is not defined', () => {
         it('follows the lang option of Marpit instance', () => {
           const $ = load(md({}).render(''))