Best to braze or TIG weld this type of repair, as you will need to apply heat to get diesel away from the area you are going to braze/weld.
No real need to worry much about cleaning the inside of the tank, as diesel fumes are highly unlikely to pose any sort of explosion risk.
I have repaired quite a few petrol tanks, and not had a single problem, but most of these have been standing unused for many years, and a good rule of thumb is that if you can smell petrol dont attempt any sort of repairs until the tank has been chemically cleaned to remove all traces of petrol vapor.
No real need to worry much about cleaning the inside of the tank, as diesel fumes are highly unlikely to pose any sort of explosion risk.
I have repaired quite a few petrol tanks, and not had a single problem, but most of these have been standing unused for many years, and a good rule of thumb is that if you can smell petrol dont attempt any sort of repairs until the tank has been chemically cleaned to remove all traces of petrol vapor.