Quickly translate or replace text

add_filter('gettext', 'translate_text');
add_filter('ngettext', 'translate_text');

function translate_text($translated) {
$translated = str_ireplace('Choose an option', 'Select', $translated);
$translated = str_ireplace('Original Text', 'Your Replacment Text', $translated);
return $translated;
}

 
Note: if the text you are trying to change is not translatable ie. not wrapped in a __( or _e( within the theme files this technique will not work on that text.

Source: https://support.woothemes.com/hc/en-us/articles/203105817-Quickly-Translate-Or-Replace-WordPress-Localized-Site-Text