Here, I will share a technique for learning language(s) in a wholistic system.

1. Acquire an old text, translated into many languages. Some suggestions are: pick a favorite Psalm or Proverb from the Bible.
2. Learn the basic pronunciation of your target language(s); one trick is to find a YouTube video reading a whole bunch of introductory sentences. Follow along and repeat.
3. Memorize your (hopefully edifying) text in all your target language(s).
4. Whenever you can, practice recalling what you have memorized. ¹

This technique is cool because it gives you a structured way to expose yourself to a lot of different grammar/vocabulary with content that you ought to be thinking of and remembering anyways.

Using the above approach for a while, you'll get a very good "feel" for your target language and can branch out to reading new texts, listening to new content, etc.

Other Aspects of Language Learning

Some aspects of languages will be particular to a cutlure, group, etc. So, if you want to speak any new language conversationally, you're going to have to learn a handful of frequent words and phrases anew. This is true even for so-called "artificial languages".

The above technique requires some text too; you may have to learn a new writing system.

Furthermore, there's non-verbal cultural stuff to learn. Between some languages, this isn't really an issue—good people will acknowledge sincerity in conduct.