Okay, so I did my best to explain the difference between Edelgard's address for Byleth (師, usually read "shi" but the furigana forces "sensei" here) and everyone else's address for Byleth (先生, read as "sensei").

Feel free to ask questions. Back to translating Edeleth C...

Also, here's a link to a Reddit thread that also covers the forms of address (the difference between 師 and 先生) that another translator wrote up, referencing a third translator. I think all three of us have a similar take, but there is more detail here. https://www.reddit.com/r/Edelgard/comments/dcypc4/the_unique_relationship_between_el_and_byleth/

Lastly, it seems that the translator who posted this thread really lamented the localization of the Edeleth S support, so it seems I have my work cut out for me... wish me luck, lol