Belgium has three official languages: Dutch, French, and German; Switzerland has four:
German, French, Italian and Romansh
. Luxembourg has three: French, German, and Luxembourgish. Also, ‘of’ is the usual preposition for a official aspect of a state (language, capital city, currency, etc).
The sentence is incorrect due to the use of the definite article ‘the’. It is important to note that Belgium has three official languages, namely Dutch, French, and German, while Switzerland has four, namely German, French, Italian, and Romansh. Luxembourg also has three official languages, which are French, German, and Luxembourgish. Additionally, when referring to an official aspect of a state such as its language, capital city, or currency, the usual preposition to use is ‘of’.
You can say
Germany and Austria have both adopted German as their official language, while Switzerland, Luxembourg, Belgium, and Liechtenstein have recognized it as one of their official languages.
In the second group, the inclusion of ‘the Italian province of Alto Adige (South Tyrol)’ would ensure comprehensiveness.
The sentence contains both ‘of’ and ‘in’, but ‘of’ appears to be the more suitable choice.
The language officially spoken in Germany, Austria, Belgium, Switzerland, Luxembourg, and Liechtenstein is German.
The TOP one is not so appropriate.