I am new to apps so my question is : I have downloaded a language translation app, how do I get it to work on a flickr site?

Where you downloaded from should be instructions on how to install and use. It depends on the specific app and your operating system, neither of which you told us making it harder for us to help.

There would detailed information given after the installation of apps.

Use Google Translate for translation of websites. Why risk installing downloaded software?

Go to my downloads folder. You will get your downloaded app. And run this.