Are pilots allowed to go to the lavatory / toilet while in flight?

They are allowed to go to the toilet while in flight like any of the passengers. But one of them needs to remain in the cockpit. I think they are allowed to swear, but not towards ATC.

They are allowed. On a 8hour flight depending on the circumstances there would be a recovery crew (an additonal crew) so they would take over if they really needed to

Badwords? If u mean swear words, towards the captain and co pilot yes but @ the ATC u cant identify correctly who said what which is the reason why they state their flight number before every radio conv.

there is always two pilots(sometimes more) on long haul flights so of course they are allowed to use the restroom if they need to. They just use the restroom nearest to the cockpit.

and the pilot and co-pilot are allowed to say "bad words" to each other and they do say "bad things" that all pilots and the ATC can hear, but it is frowned upon.

Not to long ago a pilot got fired for saying some rude things on air.

Yes, the pilots are allowed to leave the cockpit to relieve themselves. What's the alternative, have them wear Depends?

On some very long flights, the planes even carry a relief crew and some of the large planes are equipped with a crew rest area. There is one over the passenger cabin ceiling on a 777. It comes equipped with bunks where crew members can take a nap.

That is what co-pilots are for. Any person may relieve themselves during a flight.

I assume that when you say "Bad words" you are referring to politically incorrect terms. Using crude or obscene language is frowned upon in any workplace. That is what that is, a workplace. If an ATC worker is offended by the pilot or co-pilots speech than they may report them. Same goes for the Pilot or Co-Pilot to each other.

Of course they can go. As long as one pilot remains at the controls and the aircraft is in autopilot.

It doesn't matter what they say, ATC would have nothing to do with that. Pilots don't need ATC clearance to leave the cockpit.

When the plane is up flying it’s usually in autopilot and its only need one of the pilots to be in command. If one need to go and he is in command he hands control over and leaves. Same when they are having their dinner

the co-pilot takes over or the plane is put in auto-pilot. bad words are unprofessional so they are rarely used

yes he can he just puts it on auto pilot

No, you have to hold it the entire time.