A personal language is created the intention of being used, but only by one person in the null domain, that being all linguistic acts that do not require an understanding collocutor.
The features and goals of a personal language may include any known features of artificial and natural languages. Because the structure of a personal language may happen to resemble an auxiliary language, artistic language or engineered language one might confuse the goals of these languages with personal languages.
Languages adapt to the environment they’re used in. A key feature of a personal language is that it is used, and not just an sketch of how a language might be. As a language is used, features that are awkward wither away and features that were missing will spring into existence. Natural languages have a tension between efficiency and convention. It is more efficient to drop unnecessary sounds from words, have short words for commonly used concepts, but it is more important to follow the conventions of the community if one wants to be understood. A personal language would be expected to evolved to be a very good match with the user’s environment.
Other characteristics and benefits.
Highly optimized to needs of one person.
Highly but not completely private.
Standards of quality are personal. It might be a relex of English, it might be a clumsy re-hash of Sindarian. Since it isn’t written primarily as a work of art for consumption by others, the fashions of the moment are irrelevant, although one would expect to see influences from the laguage designers mileau.
There maybe benefits to creating and using ones own language beyond the benefits of having a hobby, for example, it is known that bilingual people get alzheimer’s later. If one didn’t have the opportunity to learn a popular natural language (which is a daunting task), writing one’s one language might be just as beneficial as a mental training exercise.
The pronoun system does not need to develop “you” because it wouldn’t be a very important word. 1st person and 3rd person would be more important.
Phoneticis are less important, anything will do. There is no feedback loop to fine tune the phonetic system.
If one is using their language to speak to unseen beings, then one would need a well developed pronoun system covering “you” but it wouldn’t be so personal anymore, would it? Pure personal languages aren’t for talking to anyone, seen, unseen or imaginary.
There is no particular philosophy of personal languages, but solipsism seems pertinent. If one has messages and isn’t communicating them, maybe it is because there is no one to communicate with or at the very least the other’s aren’t that important or they just don’t understand you.
Another angle is that one has information that describes reality from one’s own perspective and that is unique enough that it can’t be communicated, but is nevertheless worth recording for future use. Using a language for communicating the common deniminator messages that everyone understands would be suboptimal for recording the messages that only you understand.
What a personal language definitely is not.
It is not an auxiliary language, even if it is easy to learn and relies heavily on loan words.
It is not necessarily artisitic, naturalistic, engineered, although it might be.
It is not a community.
It is not a living language, but one could call it a moribund one, on deaths doorstep. This is okay because propagation isn’t a goal.
All that said, a personal language could potentially gain fans and even be used by people other than the creator, should the creator publish enough materials. I suppose if fans were to stay true to the philosophy of a personal language, they’d contort the language to suit their own minds and not use it to communicate with other fans or the creator.