The message of confirmation that your request has been submitted is in English even in a French Catalog (French browser). The
Popup message is : "Your request is placed successfully and is under process: TITLE (# of the request)"
The catalog request submission confirmation message should be displayed based on the language setting in the browser , for example , It should be "Votre demande a été soumise avec succès et est en cours de traitement TITLE(# of the request)") in French
Service Catalog 17.1 or 17.1 with rollup1 patch
1) Catalog Engineering team released a testfix T6D9382 to address this issue . T6D9382 is a language related patch and is an independent patch applicable on any level after 17.1 Rollup1 patch (no matter if you applied any patches after 17.1 Rollup 1) . You can open a support case to request testfix T6D9382 .
2) This language related textfix T6D9382 will be included into 17.1 Rollup 2 patch ( which will be available in September 2018 )