I modified the front-page.php file and enclosed all fields with the function __() and then in the Appearance > Customise > Guardian Options > Theme Slider Options fields, wrote [:en]Text in english[:it]Text in italian[:]
Thus,
esc_attr($wl_theme_options['slide_btn_text'])
becomes
__(esc_attr($wl_theme_options['slide_btn_text']))
and
Text in english
becomes
[:en]Text in english[:it]Text in italian[:]
And it’s all working good!
I’m sure there is a better way, but I’m not knowledgeable enough.