Almost interchangeable in British English, though: formalise is slightly preferred in general usage but both the Oxford Dictionary and the Cambridge Dictionary prefer formalize, with -ise a secondary spelling.
Organise, realise and recognise might be different. However, such words were originally spelled with the -ize ending even in the UK until -ise became more popular in the 19th century. Oxford University Press evidently still prefers to use the -ize ending for words that derive from the Greek suffix -izo (the -ise ending denoting French origin).
English is weird. I congratulate anyone who has the gumption to learn it as a second language.