Sentence comes from a code like this
Translator.translate('Please specify shipping method.')
You can take any message, for example
"Please specify shipping method."
And put a translation:
"Please specify shipping method.","Some translation"
OnePage Magento Checkout will simplify the checkout process in your store and increase your sales. By reducing the amount of steps to just a single one more of your customers will complete the purchase. Supports AJAX-updating of totals, Payment methods, Shipping methods, Shipping rates and much more. Fully responsive design.