I tested in postman…this is a working example. You are missing at least one bracket (after customerUpdate).
{"query":"mutation updateLocale($input: CustomerInput!) {\n customerUpdate(input: $input) {\n userErrors {\n field\n message\n }\n customer {\n id\n locale\n }\n }\n}","variables":{"input":{"id":"gid://shopify/Customer/20952568692758","locale":"en"}}}