.htaccess - language versions of the folder are swapped - how to configure?

Question

htaccess

Previously, the main version of the site was Russian - the files of which were in the root of the site - site.com.ua/
English (the file names are the same as the Russian version) in the folder - site.com.ua/eng/
Now the English version of the site
A is in the root Russian in the folder - site.com.ua/ru/
How to configure .htaccess correctly?
Those. so that all requests - site.com.ua/file1.html
Redirect to - site.com.ua/ru/file1.html
And vice versa requests like - site.com.ua/eng/file1.html
Now redirect to - site.com .ua/file1.html
Thank you very much!