Tonight we will have/are having dinner with the Smiths.

English Explanation:

'We are having dinner with the Smiths tonight' uses the present continuous for a future arrangement or plan. This is very common for social events. 'We will have dinner with the Smiths tonight' uses the future simple, which can indicate a decision made now or a prediction. For a pre-arranged social engagement, 'are having' is the more natural and common choice.