In French like in English, the formality of a word, or lack of, can and often do evolve during the time. Its popularity too. The TLFi sometimes considers informal what is mainstream today.
In any case, while it is useful to know the various ways to say something in French, it is better to stick the the standard way until you are very familiar with the language level of the variants and you know exactly what levels are expected during a conversation.
A typical word English speaking people have often an issue translating to French is "guy".
Here are various ways to translate it:
- Un individu : formal
- Une personne : standard
- Un monsieur : often familier, child language (m'sieu)
- Un mec : used to be argot, is now familier
- Un gonze : argot, outdated
- Un type : familier
- Un keum : argot (verlan)
Very often, I have heard foreigners using type or mec (because they have guy which is colloquial in mind) in situations where native speakers won't have used these words but just personne.
I guess this is somewhat similar to a non native English speaker writing I wanna in an e-mail.