Drops is a mobile and web application that helps to learn vocabulary for various languages. Although it is free for use in timed batches (which might be suitable for a lot of people as this type of thing can be repetitive and mundane)..
Otherwise might want to unlock it with premium access. Lifetime premium for $109.99AUD which they say is 65% off which might not be entirely accurate as it is usually around $249 AUD. Lifetime premium discounts don't seem to occur very often and seems good for those who see value in it. Premium offers extra features and allows you untimed usage.
This language app has Vietnamese with a southern accent pronunciation which is also quite rare as well and might be helpful to those seeking to learn the southern dialect.