Language is not just a method of communication, but also the heart of a country's culture. There are certain nuances a learner just cannot understand without having lived in the country for years. And even then, the individual will still have difficulty in wrapping their head and tongue around these things. Examples of such are slang words or words with subtle differences in intentions or usage. A learner must be willing to accept this fact if they are to continue learning in an efficient manner. There are words and phrases that they simply cannot have a complete grasp of.
Phrases, slang, and idioms all have intuitive differences. These differences make it difficult, if not almost impossible for non-native speakers to become fluent. There are certain differences that only native speakers who grew up around the culture can understand. This is in keeping with the previous method of accepting that fluency is too abstract a goal to pursue. It is important to remember that it is just not possible. It is something to keep in mind when a learner feels overwhelmed by the breadth of the language they are studying.
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The steps you offer, for the acquisition and mastery of a new language are right.
The comment about the jargon, also called idioms, are really those that hinder learning, being many with different concepts and changing them regularly.
Within cities, from the same country they sometimes use phrases, which are typical of there and when moving to the nearby city, the meaning is different.