Solved. Managed to put it in setup and noticed there was writing mistake when I first tried. For someone else looking for answer in similar problem, my final code looks like this:
//Setup function
function nature_light_setup() {
//Defines textdomain used in this theme
load_theme_textdomain('nature_light', get_template_directory() . '/lang' );
}
add_action('after_setup_theme', 'nature_light_setup');
