OK, I tested this in Flow and confirmed that it works. One nuance that I forgot…because Flow uses Liquid you cannot use {{ or }} anywhere or Flow will interpret it as liquid (solved by adding a space in between).
{"query":"mutation customerUpdate($input: CustomerInput!) {\n customerUpdate(input: $input) {\n userErrors {\n field\n message\n }\n }\n }", "variables":{"input":{"id":"{{ customer.id}}","locale":"fr" } } }
And just in case, this is what the config looks like:
