The following steps will fix this issue,

1. On the website, goto Customize -> HTML/CSS/JS Editor.
2. Add the following line to Web Layout section

<meta http-equiv="Content-Security-Policy" content="upgrade-insecure-requests"/>

3. Save and the menus will start appearing while switching to the secondary language 

odoo.conf

proxy_mode = True

Rgds

After some caffeine and hair pulling, I *sort of* figured out on how to resolve this issue.

Yesterday I've updated my Odoo docker container, which made the issue worst. I only have 1 additional module, which is the web_responsive module and updating that via docker is not a walk in a park. Managed to upgrade that and made the "missing Menu Bar" issue on and off. Some uninstall / reinstall of modules later and I found out this will fix my issue:

Go to Apps > eCommerce > then do Upgrade

After that the web issue seems to be fixed. Now my web Menu Bar will show the secondary language if selected. The additional web_responsive module also works like a charm so I guess it will work on other additional module(s).

Now to give myself  a pat on the back, and a lunch treat.