Freedom in acknowledgments section of dissertation - writing in native language
There are a couple directions to look to see how your school handles this sort of thing: 


Your supervisor probably has an idea of what's been done in the past. 
Theses in your school or department's library - school's keep past theses that have been accepted on record - have a look at what people in your school, department, and program have done in the past. 


In general - a quick survey of what my colleagues have used in the acknowledgements section of their master's theses has determined:


Inside jokes are ok. 
Multilingual messages are ok (we are in Canada though, so French and English is pretty common)
None of us believe the any of the referees bothered to read the personal acknowledgements.