I'm afraid the Polish language is not yet available and we don't have any information yet as to when. We appreciate you sharing your thoughts with us and we're always looking for ways to improve. Kindly send a feedback and let me know if I can assist you in anything else, otherwise I will be locking this thread in 24 hours.
Thank you for your understanding about the Polish language and I'm sorry to hear you're having some concerns about French texts shown on your Nest speaker. Let's take a look at it ― what Nest speaker do you have there? Is this the first time this has happened? Let's try this: do a sequential reboot by unplugging the power cord from your router followed by your Nest speaker. Restarting your phone might also help. When everything's back up and running again, open your Home app and go to your account on the upper right corner, tap assistant settings then select languages. If you see French, kindly remove it.
Check out this guide for more info.
Keep me posted.