The preprocessing function you want to create needs to:

Prefix the input with a prompt so T5 knows this is a translation task. Some models capable of multiple NLP tasks require prompting for specific tasks.
Tokenize the input (English) and target (French) separately because you can't tokenize French text with a tokenizer pretrained on an English vocabulary.
Truncate sequences to be no longer than the maximum length set by the max_length parameter.