J3.x:Multilingual Template Styles are Broken
From Joomla! Documentation
In Joomla 3.9.9 if you have assigned a template to a specific language (using the Default field NOT just be using menu assignments), you'll suffer an issue due to a bad database update.
You'll see one or more duplicate template styles in the template styles view of each languages template style.
- Example of template styles view with Joomla 3.9.8:
- The same view after updating to Joomla 3.9.9:
This pertains only to Joomla! version(s): 3.9.9
What is the cause
This was caused by a bad SQL change in the version. Unfortunately this bad query caused the loss of data
How to fix
- Upgrade to Joomla 3.9.10 (or later)
- Manually go back and modify the default language for the template style.
See the image below for the location for the Joomla Core Templates (Note this location may vary depending on your template provider).