Hi Ciarán,
I started using Joomla! 3.6.4 last month, so I'm sorry I don't know so much about the setting and I'm still learning by myself.
So could you please tell me how to set it in detail and to send it to you?
For your information, I did the things below.
Before setting these, the layout was OK on both PC (But the same thing happens when I shorten the width of the browser) and mobile.
1. I created a new module position in the header area of my template from your tutorial at
https://www.joomla51.com/tutorials/item/creating-a-new-module-position-in-the-header-area-of-your-template
2. I changed the setting a bit like these on Custom CSS field.
[English Page]
#header {
position: relative;
}
#languagesw {
height:50px;
margin:0;
padding:0;
position: absolute;
right:0px;
top:50px;
width:145px;
z-index:99;
}
[Japanese Page]
#header {
position: relative;
}
#languagesw {
height:50px;
margin:0;
padding:0;
position: absolute;
right:0px;
top:70px;
width:125px;
z-index:99;
}