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.

2 comments
I would like to note that "Roblox Lua" is not its own language. It's just Lua + Roblox Api
I think it is also important to note that Roblox uses Lua as a scripting language. As for learning a first programming language, I recommend Java because it is good for beginners who are learning how to use OOP for the first time.