Rövarspråket
Rövarspråket is a Swedish language game. It became popular after the books about Bill Bergson by Astrid Lindgren, where the children use it as a code, both at play and in solving actual crimes.
The formula for encoding is simple. Every consonant is doubled, and an o is inserted in-between. Vowels are left intact. It is quite possible to render the Rövarspråket version of an English word as well as a Swedish, e.g.:
that syllable chain would mean stubborn. Needless to say, the code is not very useful in written form, but it can be tough when spoken by a trained user. On the other hand, for an untrained speaker, a word or phrase can often be something of a tongue-twister or a shibboleth.
Today, the books are so well known in Sweden, and also in Norway, that the language is part of the culture of schoolchildren. Most Scandinavians are familiar with it.