Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.4.12)
// Compass (v1.0.3)
// ----

$couleur-bandeau: (fr: blue, en: red, es: #f0f);

@mixin lang-color($prop, $color-name) {
  @each $lang, $val in $color-name {
    #{'.lang-' + $lang} & {
      #{$prop}: $val;
    }
  }
}

.header {
  a {
    @include lang-color(border-bottom-color, $couleur-bandeau);
  }
}

SassMeister-output.css

.lang-fr .header a {
  border-bottom-color: blue;
}
.lang-en .header a {
  border-bottom-color: red;
}
.lang-es .header a {
  border-bottom-color: #f0f;
}