How to properly set up redirects when switching to a multilingual blog?

Question

Search Engine Optimization

Hello!
There is a blog, most of the posts are in Russian, but there are also in English.
I decided eventually to translate all Russian posts into English.
That is, to make a normal multilingual blog.
Given:
blog.ru - about 3 years ago it was the main domain, from that moment through 301 redirects leads to blog.com URL
format: https://blog.com/plc/123-cyrillic/full

I want separate languages ​​on subdomains: ru. blog.com and en.blog.com

Question: how to properly organize a redirect now, so that the current Russian-language positions would not sink ...
Until now, no optimizations have been done and it has not been distinguished in any way that there are different languages.

Let me explain the problem:
1. Many external links from foreign resources lead to Cyrillic posts. Similarly, with links from Russian-language resources. But there are also many links from Russian-language resources to English-language posts.

Let's say there is a Russian-language post blog.com/plc/123-cyrillic/
Which has been in the index for a long time.
Is it worth it automatically 301 redirect to ru.blog.com/plc/123-cyrillic/ ?
How will the search engine perceive this if the same entry WILL be in English, but with a different url: en.blog.com/plc/123-latin/ ?
Of course, rel=”alternate” will be written in the title for Russian and, if any, for English.

2. On the main blog.com it is better to auto-redirect based on IP to en. or ru. better not to, right?

I read the recommendations of Google and Yandex - there is nothing about the transition.

Answer 1

I would make language versions not on subdomains, but in folders (ru / en). For the main version it is possible without a folder, and for the other - in a folder.
If the same page (an article, for example) exists in both languages, it is advisable to link them with rel="alternate" meta tags.
To save weight from the link mass, be sure to set up redirects.
Make a list of the pages that have the most external links, plus those that have the highest traffic.
For each of them, write redirects to new urls (if the url changes, of course).
Do not get excited with auto-redirects, be very careful not to break firewood from the point of view of search engines.