Based on your description, I do not think Peter's answer of "snow bird" is particularly accurate for this situation, for two reasons:
- The woman stays in Marseille almost all of the year. She is rarely in Bordeaux.
- Google Maps is showing me that Bordeaux is barely north of Marseille. It's a decent distance away, but mostly in the east-west direction.
The typical reason for someone being a snow bird is because they want to stay where the weather is warm all year round, so they travel north or south as the seasons change, like migratory birds.
In English, I'm not sure you have a perfect phrase to capture this type of person. The best I can come up with is
That could be used as an adjective to describe her, or you could actually refer to her as "a transient."
(adj) staying only a short time
(noun) a person or thing that is transient, especially a temporary guest, boarder, laborer, or the like
David Washington's suggestion of intermittent resident is also good. A similar term you might hear is impermanent resident.
Especially in legal terms (e.g., for tax purposes), at least in the states, you would have a "permanent residence," where you theoretically would spend most of your year; the opposite would obviously be an impermanent residence, though the latter is probably not an especially common term. Still, we could extend the idea of an "impermanent residence" to describe this woman as an "impermanent resident."
One term that would be more common for the non-permanent residence would be vacation home. There is not really a matching term to describe the person who goes to the vacation home, though.