diff --git a/layouts/shortcodes/tabpane.html b/layouts/shortcodes/tabpane.html
index 4836f62675..6ce014f08d 100644
--- a/layouts/shortcodes/tabpane.html
+++ b/layouts/shortcodes/tabpane.html
@@ -59,7 +59,8 @@
     {{- end -}}
 
     <!-- Replace space and +, not valid for css selectors -->
-    {{- $lang := replaceRE "[\\s+]" "-" $lang -}}
+    <!-- Lowercase for consistency -->
+    {{- $lang := lower (replaceRE "[\\s+]" "-" $lang) -}}
 
     <li class="nav-item{{ if $rightpush }} ml-auto{{ end }}">
       <!-- Generate the IDs for the <a> and the <div> elements -->