Hi Olga,
For Issue 1, please check https://stackoverflow.com/questions/7741280/change-html-language-from-en-us-in-to-en-gb-in-wordpress/37571439#37571439. You need to set language. If you check line 27 in theme folder full-frame-pro/inc/fullframe-structure.php, there is language attribute function added. So, it you set the language as described in language, the attribute should be displayed.
For Issue 2, jQuery must be used. As far as I know, it does not have security vulnerability as it is widely used my WordPress. For more details check : jquery.com
The two sliders that you are talking about is not feasible and here are reasons why:
1. It will add two sliders, even if hiding, it will take load times
2. It will add extra images
3. Image Loader options is there. It is there in Appearance-> Customize-> Featured Slider -> Image Loader
WordPress automatically resizes different size images and will make new images of each size, that is why it would not be feasible it would only burden your server.
You can do it manually using a child theme though.
Finally, thank you for your valuable feedback, this is what keeps our themes going. All your suggestions and feedback are carefully reviewed by our team.
Please let us know if you have anything else.