he’ll find himself in a world that doesn’t care about him at all
Well we all already live in the world - including him - and how we interact with it is our choice, even if we have little to no control over what ‘might’ happen, we can at least choose to engage at whatever level we can/want.
Your bro has shut himself away and is content to ignore what he wants, that’s fine, but with that comes consequences.
If he is not capable of understanding the consequences because of a disability then that needs assessed so that he has a chance of additional help now to prepare him for the future.
And to snotty PP that means an element of being judged 😂 I mean ffs otherwise you are just enabling some waster who can’t be arsed to use up resources that those who genuinely need help could be using (or OP’s time/money/energy she could put into her own self & he already has one indulgent mum)
It is necessary to find out what actual barriers to living independedly that he might have because only then can you work out what might help overcome those barriers
So, I would be encouraging engagement with GP, to see where the land lies healthwise all round as a first step - both mother & brother - for all of the following too:
Encouraging contact with charities/support groups. In my area alone there is at least one specifically to assist living independently for those with additional needs, a couple for those ageing to help maintain (or build) independence.
There is another which runs support groups for those facing various challenges to have a place to talk out worries/get support, and an absolute constellation of smaller community groups for support, encouragement, learning and company.
There are loads of resources to learn about managing finances - myriads of courses online, on all sorts of things that will help, hell I’ll bet there’s a Youtube video on running a house - there is certainly bloody millions on cleaning & cooking.
If he can learn Russian then cognitively he could watch, enrol, engage and learn some of the above. Even if he has to watch videos in Russian to do it, or find a video game that is about keeping house? 😂
We could all wring our hands in worry and sympathy for the OP, Mum & bro, but that will do damn all to address the growing problem.
But ofc you can lead a horse to water & all that so you will have to be clear what you are prepared to do/not do if they refuse to even make the effort to help themselves first by engaging with external agencies and making whatever effort their individual capability allows.