There are two possible reasons :
Poor workmanship in the first pace. We saw a number of show homes form clef a la main builders in France where I thought the tiling was shoddy. The living room of our current house is 32 square metres and is the only room with joints you trip on.We bought the house despite the floor.
The ground / foundations have moved. Would not touch it with a barge pole unless the damage was clearly very old and I could retile from scrath
If it is a fermette and the floor you describe is over a cellar then it is a suspended slab, either concrete or perhaps shallow brick arches bridging the gaps between steels like mini railway lines spanning across the cellar walls.
In either case the tiles (which may have several other layers under) will probably be resting/floating on a bed of sand, in this instance what you describe is quite normal, in fact in areas of heavy traffic the floor can dip and rise up on either side of the "traffic lane" rather like the effect of running a plate compactor over block paving. this only becomes a problem when the undulations are such that the joints begin to open but is easily remedied by relaying in the original manner.
In your shoes I would prefer for it to be the original floor (wars and all) that has workd fine for over 100 years and would be more worried if it looked as flat as a billiard table indicating recent repair/renovation work.
If you have structural concerns you should be in the cellar looking under the floor slab for any indications.
Good luck
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