I think it is waste of time trying to learn scripting with Roblox. Of course Roblox uses Lua which is a heavily used programming language. But there are many features that exists only on Roblox.
For example, there will be no studio that supports 3D space for Lua and there will be no place to upload decals and there will be no already existing online server. Which means Lua could be a lot different from the one in Roblox. Maybe the usage of the knowledge I have gained in Roblox will be useless for other programming languages.
I don't know if it is better to learn actual programming languages to make standalone game someday or practicing it on Roblox.
Lua in particular is a great language to start with at the sacrifice of not having built-in object-oriented programming (OOP). Roblox Lua is a variant of Lua and is also great to learn as a starting language. Sure, it's a limited language but you can do quite a bit with it.
As for other languages, if you do want to have object-oriented programming as a feature in a starting language, start with Python. Otherwise, you can extend to other languages from Lua.
Well, to be honest, rbxlua is my first language and my only current language, and you can't really compare languages according to their difficulty cause each language has its own way of programming. Now rbxlua, or lua in particular is good for starting out, since it's easily comprehensible, and another good thing about going with roblox as a way of showing off your work or your game, you're stuff can get easily noticed by others (sure you might need some money to advertise your things but there are other ways of doing that).