Will localizing a tween function create a memory leak?

You can read about it here:

https://developer.roblox.com/en-us/api-reference/datatype/RBXScriptSignal

Variant RBXScriptSignal:Wait ( )
Yields the current thread until this signal is fired. Returns what was fired to the signal.

… so there simply isn’t any mechanism for cleaning up Event:Wait() calls since they don’t create any Connection objects like Connect does.

https://developer.roblox.com/en-us/api-reference/datatype/RBXScriptConnection

If you want to know what can create memory leaks, check out these posts, especially the last one: