@Ulgan That illustrates a difference of opinion in what translators should strive for. Is a translator supposed to try to make it so that the reader knows how it was written in the untranslated work/raws? If that's the case, then we can take it to the logical extreme and just tell people to look at the raws with no translations. Or is the translator supposed to convey the information and feelings that would be felt by a native reader looking at the raws?
Here, you even point out the "tone" the character uses as being an important thing to convey. That directly contradicts your preference. At this current time, on this current website, would an average Mangadex reader know that a character saying "sankyou" had a particular tone of informality and closeness? If you can honestly answer yes to that question, then fine, you're right. If you honestly think that an average English reader on Mangadex would see "sankyou" and understand the subtle differences between that and, for example, "danke" or "arigatou" or "thanks" or "thank you", then fine, you are right. Do you honestly think that is the case?
I, personally, don't. And my evidence is in the fact that other people even pointed it out. Again, I'm repeating myself, but having it as "sankyou" makes it noteworthy, makes it noticeable. I say that it does the complete opposite of the intended tone. I say that if you do care about tone, then this is not the route to take. Again, both you as well as the translator specifically cite tone as being something important. Writing "sankyou" does not accomplish the goal of conveying that tone. My position is that this action is self-defeating.
Again, not a major issue. It clearly isn't a major hurdle here. I'm just trying to show that your positions contradict what you actually want.