People learn (or actually acquire) languages naturally. As we grow up, we learn also multiple other skills which we then use in everyday life.
A lot of these skills are often compared to a language, for example:
- Playing musical instrument
- Programming languages
- Playing chess (proven to rely on patterns)
- Painting or, more generally, producing works of art.
All of the above have been compared at one point to a language. They are used to communicate ideas, emotions and information.
The language is used for communication. In the same way, one person knowing the law/programming language/accounting wouldn't actually possess any useful skill without the presence of other lawyers/computers/tax system. So one can argue that all these skills are relevant and can be evaluated only in the presence of other people who receive the message expressed using them.
In the same way, if one person invents their own language but nobody else understands it, we wouldn't classify it as a language.
So, could it be that most of the skills we learn could be described as a language? What would be arguments against it? Is there any evidence I can look into with regards to similarities between learning different skills and learning languages?