That really depends on the language and how adjacent it is to one another.

Let's say you are Dutch and learn English, then you will almost sound like a native speaker. Why? Because your mouth is used to all the sounds you make in English due to speaking Dutch and the heavy similarities between both Languages.

A German often has issues with the "th" sound because the specific way the tongue curls has to be learned.

Since I don't want to go deeper into the linguistic specifics, here is a nice tree diagram to give an idea why it's difficult to learn specific languages.

https://en.wikipedia.org/wiki/Indo-European_languages#/media/File:IndoEuropeanTree.svg