This is covered pretty clearly in the CW FAQ (I can't copy and paste right now cause I'm on mobile, but it's in there). Common words are allowed. I admittedly (and even admitted this to a submitter today) have been more lax with it than we should be, and I'm trying to be better. It doesn't matter if you can just Google it. I mean first and foremost, Google translate isn't exactly 100% accurate. But this is, again, a clearly stated rule.
Making a list of "acceptable" words isn't really feasible. So my advice is, use your best judgment. If you have to tell me the translation of the words in your submission notes, then that's a clear flag it isn't commonly used enough to work.
This is not going to be shifting over into pets and the like. That is totally separate and has been okay by the official rules for years now.
[Edit] I wanted to add in that I'm not going to go through thousands of CWs and retroactively change names! So that's not something to worry about.
It's funny you use Senpai as an example cause I had originally written in my previous response that "words like 'kawaii and sakura and senpai' weren't going to stop being accepted, haha, but I took it out. I mean a lot of people know the word senpai just from SENPAI PLEASE NOTICE ME memes, and it definitely is a word more common in colloquial usage these days.
I do not want to go into the details of the name from the aforementioned denial, but it wasn't just a casual Japanese word thrown it - it was multiple words linked together by a conjunction. Conjunctions in other languages do not make sense in English, and it was definitely going to be confusing grammar-wise for anyone reading it in English. This is the sort of thing that isn't really allowed. I hope that helps!
I want to reiterate that this thread is making it seem like I've denied dozens of items due to this issue. It still remains just one single item. I'm a bit baffled at the response here, only because it truly doesn't seem to be a major issue.
If you have an issue with it personally, please submit a ticket and I'll be glad to review it with you! If you have some specific examples in mind, I would love to hear them!
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Also, just a friendly reminder that anyone is allowed to post on a feedback thread. The forums are public and we welcome responses from every user. If you want to discuss an issue privately, the best place to do it is via a ticket :)