The first and foremost thing is checking the radio frequencies it operates on. Depending on where you live (or travel!), you might not be able to use it. Each device is different, so don't fall in love with it before you've checked the frequencies on the spec sheet.
Then, realistically, you need to make sure that apps you do want to use do still offer functioning Android builds for what's almost certainly going to be an older version of Android running on a keitei. You might really struggle with banking apps due to this (despite plenty of keitei having NFC!), it depends on your bank's policies. You might also struggle with any official government apps if they force a recent version of Android.
If it's Android, you can almost certainly run it in English, but beware any obscure outliers who heavily customize their ROMs and save space by removing other languages (few and far between, but they apparently do exist and can bite you on the ass if you don't read Japanese!).
You'll probably really struggle to make it work for you overall if you don't live in Japan where this kind of phone is still expected to be supported. There's some beautiful phones, absolutely gorgeous designs in fact, but the above are a total pain in the arse and is what's made it non-viable for me (what forced me over to a Pixel 6a on release). The Pixel 6a getting a firmware update today that affected my battery (https://support.google.com/pixelphone/answer/16340779?hl=en) is why I'm pining after another keitei, but I don't think there's anything that can tick the necessary boxes for me. I'm gutted!
There are some nice ones that have dual-screens, touchscreens, and touch-sensitive capacitive keypads for scrolling or navigation. Some can be folded to be used like a standard touch-only candybar style phone if you don't want to use the keypad. Some allow the display to be rotated, but those are very uncommon these days.
I hear Gen Z loves retro flip phones as they are less intrusive and more sensible with swappable batteries, built-in screen protection et al, so perhaps there's a potential market fit for something in the West that ticks these very specific boxes.
The funny thing is that I look at the average side-opening book-style wallet-esque case for candybar style touch phones in the West and they're basically recreating the beefits of flip phones with the hinged screen cover cases, but without much of the impact protection.
I'm not a luddite by any means, I'm not even nostalgic for flip phones in any way, I just think that I've realized there's pros and cons to both, and the current offerings available in the West don't really suit me.
Most popular phones in Japan these days in terms of units sold? iPhone and Pixel. That's what's killing the "Galapogos syndrome" keitei in terms of new models! A few friends I have in Japan find the candybar form-factor to be novel and cool, and others want larger screens for media, specific iOS-exclusive apps. or just enjoy the Apple ecosystem as they use a Mac for their work and play. They do tend to agree that there's pros and cons to both though! Pixel trails after the iPhone in numbers BTW.