"At the company, everyone can choose their office."
Translation:Dans l'entreprise, chacun peut choisir son bureau.
The confusion is in the English. Like ‘chacun’, ‘everyone’ is singular, so you could say ‘his office’ or ‘her office’. However, English allows the plural ‘their’ as a non gender specific singular. Since the gender in French refers to the object owned, not the person, the problem of gender does not arise.