Hi,
For the workflow notification email to appear in English, the Subject and Body have to be available in the approver's language, so this you will have to do.
On clicking the D365 hyperlink, the approver will always see the D365 form in their chosen preferred language.
User defined fields (such as Purchaser Requisition's name) or the comments typed when submitting to the workflow will not be translated. So the requester, would need to take that into account when creating the text ("This one needs to be approved quickly, let me make sure I word it so my english boss can understand!")
Vendor name is user defined and there is no translation table linked to it; you'd have to agree on a field to record the translated name on the Vendor master record ("Also know as" is a candidate). However, its not a field that is available as a Placeholder that could be used in the email notification ("Delivery name" is, so you could use that as an alternative). Item description has a translation (from Product) but not available as a Placeholder for workflow notification email (buit external item description is). So I suggest you look at the available placeholder for Purchase Requisition approval workflow and see how you can creatively meet your customer's requirements.
If you agree on naming convention (eg As know as, external item, ...), the from the D365 F&O web client, the English approver will be able to recognize the information (may have to modify some forms to display correctly). If absolutely necessary, you may want to consider adding some translation tables for those Master Data elements that have none.
For workflow notification email, I would not suggest modifying the fields available as placeholders. Rather, I would recommend using PowerAutomate which has more flexibility and where you can query the translation tables to pull the information in the approver's langage.
Hope that helps!