When do I say I can only start PhD late
(European professor with hiring experience here.)
A month and a half is not a significant delay, but a quite moderate one. If the professor wants to hire you, it's rather likely that they would be happy to have you already one and a half months after the planned start date! So I recommend to not address it before you have an offer or they ask.
More generally, it's understood that people take a few months to start a new position, even more so in academia, where people typically move between countries for a job. That applies even stronger in the times of a global pandemic, where visa issues alone can defer the start date by many months.