Append language abbreviation (EN, RU, PL) in superscript to a link if the language of the page is different.

hreflang-conditional-link-tag.scss

@mixin linksignify-superscript {
  font-size: 0.83em;
  line-height: 0.5em;
  vertical-align: baseline;
  position: relative;
  top: -0.4em;
  left: 0.1em;
}

// Append language abbreviation (EN, RU, PL) in superscript to a link if the language of the page is different.
html:not([lang^='en']) {
  a[hreflang^='en']:after {
    content: 'en';
    text-transform: uppercase;
    @include linksignify-superscript;
  }
}

html:not([lang^='ru']) {
  a[hreflang^='ru']:after {
    content: 'ru';
    text-transform: uppercase;
    @include linksignify-superscript;
  }
}

html:not([lang^='pl']) {
  a[hreflang^='pl']:after {
    content: 'pl';
    text-transform: uppercase;
    @include linksignify-superscript;
  }
}

CSS selectors that look like regular expressions affect performance:

https://github.com/stubbornella/csslint/wiki/Disallow-selectors-that-look-like-regular-expressions

If you do not plan to specify 'lang' and 'hreflang' as "en_US" or "fr_FR", it's better to go with simple check. If you need full language id support, look at Revision 1