> Do you specifically want to support changing the language of seed
words, while keeping the bip32 root seed they generate unchanged?
What is the usecase for this?

Yes and no. Yes that the inter-operability will be much better if the
same seed could be recorded as in English and in other languages
as well. There could be a standalone 3rd party tool to convert
mnemonic sentences from one language to another so wallets
do not have to support all languages. This, in reality, does not make
any wallet more complicated or increase any dev costs.

No that I don't think anyone would switch language for fun.

And I think the symmetric function in SLIP-0039 is a good
idea to help with migration the existing wallets. So this will look like:

Chinese
English
Japanese                      <=> entropy <=> sym. KDF <==> BIP32 seed
......
Decimal numbers

>If changing the language of seeds is not a requirement, then look
into Electrum seeds. They are language/wordlist agnostic.

So, changing the language is not a requirement. However, the current BIP-39
makes the seed dependent on the language. And I'd like to see it changed.
The change will not further complicates the wallets as we can rely on 3rd
conversion tools to handle different languages.

One additional benefit multiple language support could bring up is, we can
write down the mnemonic sentence in two or more different languages
simultaneously, as a way to do checksum each other.

Regarding Jon's comments:
>lack of localized words for recovery phrase causes more money loss than
supporting it.

This also worries me.

Think of the situation that one day, crypto currencies are all over the
world.
Everyone has some coins. Among them many just cannot handle English.
We must provide a way for them to master their crypto assets. Therefore,
as Jon had stressed, it is very important to support multiple languages.
I really don't like it if only English could be used.

And then think of those who do not have a native language wordlist. Allowing
representing the seed in decimal numbers will be helpful.

Thanks,
Weiji