“Gurks” is a natural language I invented between the ages of 10 and 15. It draws inspiration from Dutch, English, German, French, Swedish and Czech, roughly in that order, mixed with my own sense of crazy sounds transliterated into characters.
The main vehicle for the language's development was a course, patterned on typical foreign language courses at the time. I rewrote the course up to four times (no computer at the time, all handwritten and illustrated). Available in PDF are the 4th edition of Mep Grurt Gurks Rummel (21MB, “I want to speak gurks”) and it's companion dictionary (3MB, 2000 or so words). Warning: all the translation in the course is to dutch only, so until I ever get around to translating it, it will be hard to learn for most people.
I designed 2 additional languages, Rors (which was like Gurks except had newly designed symbols instead of roman characters) and Frors (which had a single symbol per word). I'll get around scanning those some day.
From what I remember, besides myself, only 2 people ever made a serious effort to learn the language and converse with me, which were my sister and a childhood friend :)