Conditional loading Google fonts

I’m working on a new theme and I’d like for translators to be able to deactivate Google fonts if the fonts don’t support the translator’s language. I’ve got this function:

function mytheme_fonts_url() {
  $fonts_url = '';

  /* translators: If there are characters in your language that aren't supported by Titillium Web, translate this to 'off'. Do not translate this into your own language. */
  $titillium = _x( 'on', 'Use Titillium Web font: on or off', 'mytheme' );

  /* translators: If there are characters in your language that aren't supported by PT Sans, translate this to 'off'. Do not translate this into your own language. */
  $pt_sans = _x( 'on', 'Use PT Sans font: on or off', 'mytheme' );

  if ( 'off' !== $titillium || 'off' !== $pt_sans ) :
    $font_families = array();

    if ( 'off' !== $titillium ) :
      $font_families[] = 'Titillium+Web:400,400italic,700,700italic';
    endif;

    if ( 'off' !== $pt_sans ) :
      $font_families[] = 'PT+Sans:400,400italic,700,700italic';
    endif;

    $query_args = array(
      'family' => urlencode( implode( '|', $font_families ) ),
      'subset' => urlencode( 'latin,latin-ext' ),
    );

    $fonts_url = add_query_arg( $query_args, '//fonts.googleapis.com/css' );
    return $fonts_url;
  endif;
}

And I load the fonts like this:

function mytheme_scripts() {
  wp_enqueue_style( 'mytheme-google-fonts', mytheme_fonts_url(), array(), null );
}
add_action( 'wp_enqueue_scripts', 'mytheme_scripts' );

However, whenever I load the new theme, I get a 400 error when trying to load the Google fonts. What am I missing?