:lang()

*:lang(en-US) {
  outline: 2px solid deeppink;
}

<p lang="en-US">
  Music during road trips and long commutes aren’t a problem, but using
  headphones isn’t the best thing to do while driving in your car.
</p>

<p lang="pl-PL">
  Gdy widzimy znak z narysowaną czaszką i napisem
  <strong lang="en-US">DANGER</strong> to lepiej nie wchodzić do środka.
</p>

:lang(<language-code>) {
  /* ... */
}

<div lang="en">
  <q>This English quote has a <q>nested</q> quote inside.</q>
</div>
<div lang="fr">
  <q>This French quote has a <q>nested</q> quote inside.</q>
</div>
<div lang="de">
  <q>This German quote has a <q>nested</q> quote inside.</q>
</div>

:lang(en) > q {
  quotes: "\201C" "\201D" "\2018" "\2019";
}
:lang(fr) > q {
  quotes: "« " " »";
}
:lang(de) > q {
  quotes: "»" "«" "\2039" "\203A";
}