Is the nation the only form of collectivisation that works on a large scale?Watch
If so should they embrace brexit and stand up to against free(ish) movement of labour and capital on a global scale. People need to stay where they are are and left wingers in each country need to focus on their own countries rather than trying to create some kind of international?
However, yes it is easier to unite people based on nationalism, heritage, ethnicity and genuine shared culture than it us to unite them based on political unions which only really make sense geographically
The thing is it is impossible for labour to 'embrace' brexit fully as this would isolate and demoralise so many members as it is a core belief of many modern labour supporters that countries less fortunate should be helped at every opportunity even if it is a detriment to the UK in someway
In terms of left wingers in each country focusing on their own countries well this is unlikely to happen as many countries have stricter and less mobile regimes than the UK while at the same time being less economically stable and constantly on the periphery of conflict, so many will continue to see migrating to the UK as their best option.