Ok, if you've done many multilingual sites before then you are more of an expert than I am.
Please let me remind you of the support policy here https://www.jomres.net/support-policy
The Leohtian template is a gift, a freebie. I don't charge for it, it's purpose is to give you something to work with, nothing more and I don't provide support for it. I strictly only provide support for Jomres itself, that's already enough work.
You're welcome to use another template (or template framework, if that's your bag) if you're familiar with another system. You're absolutely not forced to use Leohtian but if you use another template based on another version of Bootstrap and not BS3 then please remember to visit Admin > Jomres > Settings > Site Configuration > Misc tab and tell Jomres to use the appropriate template set, for example Bootstrap 2.
The problem you are experiencing is not with Jomres. Jomres doesn't do anything with languages except use it's own language strings depending on the language defined in the URL, as you can see herehttps://github.com/WoollyinWalesIT/jomres/blob/master/libraries/jomres/classes/jomres_language.class.php#L38
Once it knows the active language, it choose the appropriate language file. That's it.
If you're having a problem with the redirection URL once you have added a new language then ergo is must be something to do with your configuration.
Given that T3 is a framework, I did some googling for you and found these results :http://www.t3-framework.org/documentation/bs3-customization#multiple-languagehttps://joomlaux.com/blog/item/71-how-to-create-multi-language-for-t3-framework-template.htmlhttps://tutorial.joomlageek.com/T3/how-to-use-multi-language-joomla-3-t3-framework
Hopefully they will be helpful for you.
As I said earlier, please remember that you're not forced to use Leohtian. There are plenty of other free templates you can use if you want, if you're more comfortable with them.