Why the Swiss speak one language and write another.
Diglossia, described by Charles Ferguson in 1959, means one community uses two varieties with fixed roles. The H variety is codified and used for school, news and literature, often with no native speakers. The L variety is everyday speech. Examples: Standard German and Swiss German, Standard Arabic and the dialects, French and Haitian Creole. Greece ended its diglossia in 1976.
Useful: Learning Arabic, or German for Switzerland, first work out which of the two varieties you actually need.
Source: en.wikipedia.org