Without hreflang, Google may mix up your language versions — e.g. show the English variant to a Russian user. With correct hreflang each user sees the version relevant to their locale.
Example
<link rel="alternate" hreflang="ru" href="https://example.com/ru/article" />
<link rel="alternate" hreflang="en" href="https://example.com/en/article" />
<link rel="alternate" hreflang="x-default" href="https://example.com/en/article" />