How to correctly indicate to Google that links are for multilingualism?

Question

Search Engine Optimization

For multilingual site we use separate domains for each language. And on each page we show a link to other languages ​​(domains).
We also use:

<link rel="alternate" hreflang="lang_code" href="url_of_page" />

As recommended by Google .
We have a lot of pages on each site. And I noticed that after adding links to other languages, traffic from Google fell to some languages ​​(domains).

• What to do in such situations?
  
• Maybe there is some attribute that can be used to indicate to search engines that these links are for languages?