This B737, begins rid of fuel to take off in VLC or so it seems …
B737 Air Europa Losing Fuel at Take Off - YouTube)
it may be other liquid?
It’s not fuel. It’s condensation.
Condensation is possible but you don’t normally see it prior to rotation and before the wings develop a lot of lift. You also see condensation in several areas not just one.
The 737 does not have a fuel dump system so a stuck dump valve is out of the question. The other possibility is an overfilled wing and we are seeing a bit of overflow out of the NACA vent on that side. The takeoff run seemed long indicating a heavy flight.
That was more liquid than I would expect out of a hydraulic leak. There are no other liquids in the wing other than fuel and hydraulics.
You’re right, a pilot on another forum explained that it is an overpressure valve fuel must be poorly regulated.
That is the reason that allows fuel flow only in the takeoff when an overpressure is shown