I am working in Mac - Pages program, and when I choose a template for a flyer or poster it opens in another language. How do I get it to open in english?

Are you saying the "sample text" in the template is in a foreign language, or are you indicating the template somehow changes all your settings and menus and all to another language?

If it's just the sample text, don't fret it. It's sample text, probably the same Lorem Ipsum text that's been in use for 500 years as placeholder text.

Just select it and replace it with your content. You're going to do that anyway, right?

http://en.wikipedia.org/wiki/Lorem_ipsum