To unlock the achievement "National company", the player needs to own all the garages located in the country of one's "headquarter".
The headquarter car be defined as the first garage the player choose when starting a profile.
How to change my headquarter ?
There is currently no way to change your headquarter in the game, which means you'll have to purchase each garage in each city of your headquarter country, with a 100 000€ fare.
How to complete the achievement easily ?
It is recommended to start a new profile, to select Luxembourg (city) as headquarters. Indeed, Luxembourg only features one garage, which means you will unlock the achievement instantly, without the need to purchase another garage.