diff --git a/Rakefile b/Rakefile
index 74a84954d6..8fae8a9958 100644
--- a/Rakefile
+++ b/Rakefile
@@ -14,7 +14,7 @@ CONFIG = "_config.yml"
 task default: [:build]
 
 desc "Run tests (test-linter, lint, build)"
-task test: %i[test-news-plugin test-linter lint build]
+task test: %i[test-news-plugin test-html-lang-plugin test-linter lint build]
 
 desc "Build the Jekyll site"
 task :build do
@@ -129,3 +129,11 @@ Rake::TestTask.new(:"test-news-plugin") do |t|
   t.test_files = FileList['test/test_plugin_news.rb']
   t.verbose = true
 end
+
+require "rake/testtask"
+Rake::TestTask.new(:"test-html-lang-plugin") do |t|
+  t.description = "Run tests for the HTML language plugin"
+  t.libs = ["test"]
+  t.test_files = FileList['test/test_plugin_html_lang.rb']
+  t.verbose = true
+end
diff --git a/_includes/language_selector.html b/_includes/language_selector.html
index dab2919d9a..7dc903a2f3 100644
--- a/_includes/language_selector.html
+++ b/_includes/language_selector.html
@@ -17,9 +17,9 @@
   <button class="flex items-center gap-1 px-3 py-1.5 text-sm border border-stone-300 dark:border-stone-600 rounded-md hover:bg-stone-100 dark:hover:bg-stone-800 transition-colors">
     <span class="text-stone-700 dark:text-stone-300">
       {% if current_language %}
-        {{ current_language.native_name }}<span class="hidden sm:inline"> ({{ current_language.code }})</span>
+      {{ current_language.native_name }}<span class="hidden sm:inline"> (<span lang="und-Latn">{{ current_language.code }}</span>)</span>
       {% else %}
-        English<span class="hidden sm:inline"> (en)</span>
+      <span lang="en">English<span class="hidden sm:inline"> (en)</span></span>
       {% endif %}
     </span>
     <span class="icon-dropdown text-base" aria-hidden="true"></span>
diff --git a/stylesheets/compiled.css b/stylesheets/compiled.css
index 0539c6d70b..3c69f2981b 100644
--- a/stylesheets/compiled.css
+++ b/stylesheets/compiled.css
@@ -1984,6 +1984,14 @@ body:is(.dark *){
   color: rgb(250 250 249 / var(--tw-text-opacity, 1));
 }
 
+[lang]:lang(ja),[lang]:lang(ko),[lang]:lang(zh-CN),[lang]:lang(zh-TW) {
+  font-family: "Plus Jakarta Sans", var(--noto-sans-subset), -apple-system, BlinkMacSystemFont, sans-serif;
+}
+
+[lang]{
+  font-family: "Plus Jakarta Sans", -apple-system, BlinkMacSystemFont, sans-serif;
+}
+
 /* CJK fonts */
 
 html:lang(ja),
diff --git a/stylesheets/tailwind.css b/stylesheets/tailwind.css
index 40bd2d9522..d2a19a7b87 100644
--- a/stylesheets/tailwind.css
+++ b/stylesheets/tailwind.css
@@ -24,6 +24,10 @@
     @apply dark:bg-stone-900 dark:text-stone-50;
   }
 
+  [lang] {
+    @apply font-default;
+  }
+
   /* CJK fonts */
   html:lang(ja),
   body:lang(ja),
diff --git a/test/test_plugin_html_lang.rb b/test/test_plugin_html_lang.rb
new file mode 100644
index 0000000000..fe44575ec1
--- /dev/null
+++ b/test/test_plugin_html_lang.rb
@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+require "helper"
+require "liquid"
+require_relative "../_plugins/html_lang"
+
+describe Jekyll::HtmlLangFilter do
+  include Jekyll::HtmlLangFilter
+
+  describe "#to_html_lang" do
+    it "returns the same string if no underscore is present" do
+      _(to_html_lang("en")).must_equal "en"
+      _(to_html_lang("fr")).must_equal "fr"
+    end
+
+    it "converts Jekyll locale with underscore to BCP 47 format" do
+      _(to_html_lang("zh_cn")).must_equal "zh-CN"
+      _(to_html_lang("zh_tw")).must_equal "zh-TW"
+      _(to_html_lang("pt_br")).must_equal "pt-BR"
+    end
+
+    it "returns the input as is if it is not a string" do
+      _(to_html_lang(nil)).must_be_nil
+      _(to_html_lang(123)).must_equal 123
+    end
+  end
+
+  describe "integration with Liquid" do
+    it "is registered as a liquid filter" do
+      template = Liquid::Template.parse("{{ 'zh_cn' | to_html_lang }}")
+      _(template.render).must_equal "zh-CN"
+    end
+
+    it "works correctly in a liquid template for simple lang" do
+      template = Liquid::Template.parse("{{ 'en' | to_html_lang }}")
+      _(template.render).must_equal "en"
+    end
+  end
+end