This is very sad.
The number one problem in Japan with BIP39 seeds is with English words.
I have seen a 60 year old Japanese man writing down his phrase (because he kept on failing recovery), and watched him write down "aneter" for "amateur"...
So instead I had him use Copay which generates Japanese words, wrote it down 20x faster, and perfectly. Was able to recovery on the first try.
Imagine if I didn't tell him to try recovery before using it? (iirc Trezor doesn't say to wipe and recover before using???)
If you understand English and can spell, you read a word, your brain processes the word, and you can spell it on your own when writing down.
Not many Japanese people can do that, so they need to copy letter for letter, taking a long time, and still messing up on occasion.
Even native English speakers who can't spell can mess it up badly too.
To be honest, a key storage format that doesn't support multiple languages is much more dangerous than any doomsday situation you can think of for supporting them.
BIP39 states that seed derivation is INDEPENDENT of wordlists, and that failure to verify checksum (not knowing the wordlist falls under this) should "WARN" the user and not fail, continuing to derive the seed anyways.
Currently the only wallet I know of following this part of the BIP is, ironically Electrum. I can recover any BIP39 phrase from any wordlist even if Electrum doesn't know it.
I really hope you reconsider multi-language support for everything moving forward.
I understand it's a nightmare to plan for and support, which is fine if you were just developing a piece of software sold by a company based in a western country... but you are trying to make a standard for an international currency. Defining "everyone should only use English, because ASCII is easier to plan for" is not a good way to move forward as a currency.
I am just thinking of all the users I will have to help out down the road when they come crying to me saying they can't recover, and it turns out they wrote down some non-English gibberish in roman characters claiming "I wrote the English just as it was on the screen!" and I have to write a brute force script to try all the word combinations for the mystery words. (I have done this before)
Just my two cents. Not to be accusatory or anything.
Please reconsider. Thanks.