I would first test a specific alkaline tile cleaner , we have Wexa or LTP do grimex or similar
Dilute , work over 3-4 tiles only with a white pad and brush 10-12 mins scrubbing every 1-2 mins rinse and repeat or strengthen dilution and re-test,
If you think there is old sealers applied, or a linseed oil , test a solvent based stripper.
Once test tiles clean up, work in small controlled sections to complete the floor.
If any paint drips, carpet glue,deeper stains remain , you may need to re-work or test a solvent stripper on these tile.
If after this you may have to deal with light salt residue or cement haze, you would test a specific cement residue remover- ( acidic product) again test - dilute- clean and rinse. This should cover most contamination you may have.
I used to leave mine 4-5 days minimum or until fully dry and released of all moisture after the last rinse , then impregnate.
If you wanted to seal once all work is complete, you will then need a specific Impregnator for unglazed ceramics . Matt finish - water and oil reppelance and easier to maintain.