Can you pay with Litecoin anywhere you can pay with Bitcoin? Can you pay with Litecoin at any place that accepts Bitcoin? If no where can you spend Litecoins?
Merchants that only accept Bitcoin, will not accept Litecoin directly.  

However you can pay any Bitcoin merchant with a 3rd party that automatically converts your Litecoin to Bitcoin. The most well known service of this type is shapeshift.io

Merchants can integrate their "shifty" button to make this easier for consumers to use, but the Shapeshift service is available for every consumer wishing to pay a Bitcoin merchant regardless of if they have integrating the Shapeshift API or not.

https://shapeshift.io/shifty_button.html

https://shapeshift.io/#/coins