1. https://www.shamimsplugins.com/docs/front-end-pm-pro/getting-started-2/translate/#loco_translation (see 3 steps under loco translate heading, this should include any string if missing)
3. If you not add any code also, this has an upper limit of 100 characters by default. can you send message if more than 100 characters are there in subject field? which code you have pasted?