Thanks @Don
I forgot to mention that plugin is not installed and that I tried already to change the data-template-name to something a random UNIQUE-NAME with the same result or not showing my banner at all if I made up the PLUGIN-OUTLET-NAME.
You can probably tell by now , I’m not familiar at all with handlebars/ember 
My understanding is that I’m customizing a template that has it’s predefined place in the html and has a result the custom html is not anymore over /html/body/section main but deep inside resulting in css inheritance I didn’t have before.
What I struggle to understand is Why do I need to customize any template to use a translation…
I managed to identify the template to customize using Ember inspector as advised here
And thanks to your answer and link about the plugin-outlet I found the right data-template-name=“/connectors/above-site-header/my-navbar”
Thanks again for your help