It's not really that difficult, an official language OF a country is recognized at a national level. Thus all official government communication must be issued in that language. In the Netherlands, only Dutch has that level of recognition. Same in Italy for Italian
Then there are other, regionally-rocognized language that local governments use alongside the national one (West Frysian in Friesland, German in South Tyrol, etc.), and may even enjoy a majority of speakers within those regions, but they are not "an official language OF" the wider country.
Then there are other, regionally-rocognized language that local governments use alongside the national one (West Frysian in Friesland, German in South Tyrol, etc.), and may even enjoy a majority of speakers within those regions, but they are not "an official language OF" the wider country.