Where can I translate strings from wp-login page like “USERNAME OR EMAIL”, “ERROR: Invalid username.” and etc. I could not find language file which contains these strings. Plsease see the image attached.</span>
https://drive.google.com/open?id=0B2acIOAP0GVYcEJWbnJUeHZrWVk
These strings are coming from WordPress directly, WPLMS does not modify the default WP login screen.
You will need to check and make sure that the wordpress translation files exist in your language and these strings are already translated in the translation files.