<header class="header">
  <div class="header__logo">...</div>
  <nav class="nav nav_main">...</nav>
  <nav class="nav nav_lang-switch">...</nav>
  <div class="header__phone-link"></div>
</header>

<header class="header header_fixed">
  <div class="header__logo">...</div>
  <nav class="nav nav_main">...</nav>
  <nav class="nav nav_lang-switch">...</nav>
  <div class="header__phone-link"></div>
</header>

.header_fixed .nav_main {...}
.header_fixed .lang-switch__icon {...}
.header_fixed .header__phone-link {...}
...