Which language is better and easier to learn to create a dashboard?

No language is better. They are all equal, except the ones that dont exist anymore.

you should search a theme in bootstrap or laravel have already a backend included
after that you can transform your current html website to the new theme
and you will benefit the backend as a bonus
i think this is the easiest method for you

to develop a dashboard i prefer core ui, there is a wonderfull ui with a lot of widgets, you can combine it with vue, laravel, angular or the framework that you want.

obrigado, vou tentar fazer o download quando tiver o direito de fazer

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I think metronic is the best theme i have used so far.
The Worlds #1 HTML Bootstrap 4, Angular 10, VueJS, React
& Laravel Admin Theme of All Time
Save thousands to millions of dollars by using a single tool for different types of unique and amazing admin dashboard solutions for all your projects.

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PHP is a general-purpose scripting language especially suited to web development.
It was originally created by Danish-Canadian programmer Rasmus Lerdorf in 1994.
The PHP reference implementation is now produced by The PHP Group.

use React b cuz It is now the best framework that can be
It is both ranked #1 and powerful
And as perfect as it can be

I believe that all languages are the same, it's not the language that matters, but how you work on it.

I'm struggling to see how you would not want to go with the classics: PHP / MySQL / HTML5