Website localization when already indexed on search engines

Question

I found lots of topics on how to localize a website and most common solutions have been adding subdomains or creating different subdirectories(eg. "/en/"). However I could not find anything that worried about loosing Google indexing for sites that originally were localized for only one language.

Since now, Google managed to index pages like this:

http://website.com/threads/this-is-the-title/11111

Whenever I decide to opt for localizations in different sub-directories, it would be:

http://website.com/en/threads/this-is-the-title/11111

What will happen to the hundreads of pages index by Google? Can you help me figuring out a solution to localize a website without having trouble with Google?

What I found that partially solves the problem

Hreflang: https://support.google.com/webmasters/answer/189077?hl=en

This would work except for the fact that I'll find myself with localization on two different folder levels:

/
    ...files of already index content

    /en
           ...files of the second language

Update:

Current htaccess file:

RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d

RewriteRule ^threads/(?:([a-zA-Z0-9-_]+\/)?)([0-9]+)$ thread.php?qid=$2 [QSA,L]
...other

I turned that line into:

RewriteRule ^(?:([a-zA-Z]+\/)?)threads/(?:([a-zA-Z0-9-_]+\/)?)([0-9]+)$ $1/thread.php?qid=$3 [QSA,L]

This is not enough, since it does not redirect to a localized sub-directory.

Answer 1

Use .htaccess and mod-rewirte to tell google that the url has moved to another uri

for example

RewriteEngine On
RewriteRule ^oldsite\.html$ /newsite.html [R=301,L]

And google is fine with you