Google Fonts Subsets

To implement Latin Extended subsets go to  /theme_folder/includes/custom-styles.php and check out where the fonts enqueue calls are made. One should look like this:

if ( ! isset( $f_type_1['default'] ) ) {      wp_enqueue_style( $f_type_1['css-name'], "$protocol://" . $f_type_1['css-name'] . ":300,400,400italic,500,600,700,700,800,900" ); }

Replace the above with the following:

if ( ! isset( $f_type_1['default'] ) ) {      wp_enqueue_style( $f_type_1['css-name'], "$protocol://" . $f_type_1['css-name'] . ":300,400,400italic,500,600,700,700,800,900&subset=latin,cyrillic-ext,cyrillic " ); }

Notice the subset added at the end of the call. Modify it to include the desired subset, as you see in Google Fonts app.

Unfortunately, this is a change which you'll have to apply after each theme update.