worst, you can mix them if they are just a sailor with nationality for leveling up.

But they lose curtain ability, for example you use km eng in IJN they will only use of their 30% ability ( not sure about exact ability) so its better to use IJN eng alone.

sorry if ther is wrong info on that but its something like that !!! Hehehe

On IJN ships your KM supports will perform quite a bit worse than IJN supports.

Can they still reach the speed,repair cap?

They will lose alot of ability.

There are penalties for mixing nationalities, but I think that some nations can mix without losing as much. However, even if that still applies, it's still a big enough loss to not be able to build a whole crew with supports of another nationality.

When you use a support sailor on an allied nation ship only the experts will contribute to the ability. You can compare how efficently they are working by comparing the SD they give on a KM ship then the SD they give on an IJN ship.

say i use a  lvl75 ebve eng KM  on a UK ship say till lvl 80 then put back  on KM ship is all ok or has the sailor  lost any ability that way  in lvling 4 lvls