Is it safe to take FR235 inside dry sauna ?

Manual says operating temperature is 23 to 122 degrees Fahrenheit.

I wouldn't risk it. But it'll probably be fine.

I think, generally speaking, it'll be fine if you took it in a couple of times but it sounds like you'll be subjecting it to the sauna quite often. In this case long term degredation would probably occur. It'll all depend on the heat sinking (relative cooling) your arm would provide and just how hot the critical components inside the watch could get in that time.

The general electronics and plastics would be fine, but I'd be worried about the LCD and the battery.

Try it for a short time and them immediately see how hot the watch watch got to. If it doesn't feel hot to the touch it may be ok. But again, it's not worth the risk

Don't rely on the device closest to the user (i.e. the watch) to do the filtering

Can I choose which apps are showed in notifications ? for example alert me only if I get a text or a call not every time someone likes my instagram picture ....

You make those choices on the mobile device from which the notifications are originated, and not on the watch which simply displays every notification the mobile device sends to it.

I could take it off but the thing is I don't want to keep taking it off and putting it on if I don't have to
To question is will 10 minutes in 90 ° Damage the watch at all ?

You don't have to take the watch off if you don't want to. However, you cannot have what you want, which seems to be some sort of guarantee that your preferences will not cost you or risk some sort of adverse impact.

I think it's folly to look to fellow customers for assurance. As TMK17 has rightly pointed out, the use case is well outside of the operating conditions published for the device, so any faults or damage resulting from it (or any faults or damage that can even be remotely attributed to it, if there is physical evidence that the watch was subjected to such, even if the true cause of lies elsewhere) will not be covered by warranty. Of course, it's your prerogative as the customer to choose convenience over risk minimisation.

You can also choose other methods of risk minimisation, such as wrapping a cold wet towel around your watch (and wrist) when you go into the sauna for ten minutes, but again it has to come at a ‘cost’ to your convenience. It's your compromise to make.