1751, from earlier term colloquy(“a conversation”), from Latincolloquium(“conference, conversation”), from con-(“together”) + loquor(“to speak”), + -al.
You're using too many colloquial words in this cover letter: I suggest changing "I picked up loads of cool skills" to "I acquired many positive abilities"
The colloquial, and at times sarcastic, tone of her books makes her popular with teenagers.
It is a common misconception that colloquial somehow denotes "local" or a word being "regional". This is not the case; the word root for colloquial is related to locution, not location. A more appropriate word for describing "local" or "regional" language is vernacular.
Note that while colloquy and colloquium refer to formal conversation, colloquial refers instead specifically to informal conversation (casual tone). This does not always mean spoken as opposed to written, but it always means informal as opposed to formal.