On the KLM website, I found this strange quote:
If planes pass each other at night they sometimes use light signals to say hello.
Is it true and common? How it is done (which lights/patterns)?
Yes, it is true. When two aircraft flying in opposite directions pass each other at night, it is quite common for one or both to briefly turn on their landings lights. This is done partially to "say hello", and also to increase situational awareness. I would guess it happens about 50% of the time.
Usually one aircraft will turn on their landing lights about 2-5 miles back and wait for the other to briefly flash their lights in response. It is not uncommon to get no response because the pilots on the other aircraft may have their heads down, and not see the other aircraft. Some pilots don't respond because they feel it is a useless practice, and can't be bothered.
When there is only 1,000' vertical separation of opposite direction, or crossing traffic, some ATC jurisdictions might require the controller to inform the aircraft of each other's presence. In that case it is more likely that both aircraft will turn on their landing lights to be more visible to the other aircraft. There is no operational requirement to do this, but most pilots tend to go along with this practice.