The etiquette is the same everywhere in Switzerland, regardless of which of the country's language regions you're in: keep the dog on a short leash, positioned beside or under the table rather than wandering, and never on a chair or the table itself.
Because Switzerland has four national languages, it's genuinely useful to know the question in more than one. In German-speaking areas, ask 'Darf der Hund mit rein?' In French-speaking areas, 'Le chien peut-il venir?' And in Italian-speaking Ticino, the same phrase used across the border in Italy works fine: 'Il cane può venire?'
If the answer is yes, bringing a small travel mat or blanket for the dog to settle on is a small touch that Swiss staff — and other diners — tend to appreciate, especially on a stone terrace floor.