1. Of a tyre, to become low in air so that there is not enough in it. English = go flat, flatten, deflate; (be) flat.
2. Of a liquid, especially a drink, to go from being fizzy with lots of bubbles to having no bubbles at all. It is better with bubbles. English = go flat, (be) flat.
3. Of a liquid, especially a drink, to have no bubbles in it at all because it is made that way. English = (be) still.