Add language button and remove all others

_config.yml

@@ -54,6 +54,7 @@ sphinx:
   local_extensions:
     slide_meta: "extensions"
     # thebe_local: "extensions"
+    fix_buttons: "extensions"
   config:
     html_theme_options:
       use_fullscreen_button: false

extensions/fix_buttons/__init__.py

@@ -0,0 +1,50 @@
+# import json
+from pathlib import Path
+
+from sphinx.util import logging
+# from docutils import nodes
+
+__version__ = "0.1"
+
+def add_static_sources(app):
+    static_path = Path(__file__).parent / "static"
+    app.config.html_static_path.append(static_path.as_posix())
+
+
+def remove_buttons(app, pagename, _templatename, context, _doctree):
+    if not pagename.startswith("U0"):
+        # not one of the units, don't do anything
+        return
+
+    is_engish = pagename.endswith(".en")
+
+    buttons = context["header_buttons"]
+
+    keep = []
+    for button in buttons:
+        try:
+            if button["tooltip"] == "Start presenting":
+                keep.append(button)
+        except Exception as _e:
+            pass
+    
+    keep.append({
+        "type": "javascript",
+        "javascript": "switchLanguage()",
+        "tooltip": "Deutsch" if is_engish else "English",
+        "icon": "fas fa-language",
+    })
+    app.add_js_file("switch-language.js")
+
+    context["header_buttons"] = keep
+
+
+def setup(app):
+    app.connect("builder-inited", add_static_sources)
+    app.connect("html-page-context", remove_buttons, priority=9999)
+
+    return {
+        "version": __version__,
+        "parallel_read_safe": True,
+        "parallel_write_safe": True,
+    }

extensions/fix_buttons/static/switch-language.js

@@ -0,0 +1,19 @@
+const switchLanguage = () => {
+  // Get current URL
+  let url = window.location.href;
+  let path = window.location.pathname;
+  let filename = path.split("/").pop();
+
+  // Detect if it's an English page
+  if (filename.endsWith(".en.html")) {
+    // Switch to German: remove ".en"
+    let newFilename = filename.replace(".en.html", ".html");
+    let newUrl = url.replace(filename, newFilename);
+    window.location.href = newUrl;
+  } else if (filename.endsWith(".html")) {
+    // Switch to English: add ".en"
+    let newFilename = filename.replace(".html", ".en.html");
+    let newUrl = url.replace(filename, newFilename);
+    window.location.href = newUrl;
+  }
+}