I think this question needs some clarification.  Hopefully whoever asked this will see the post and be able to add some more context.

Unless the question simply relates to changing the project template to only use one language pair when the project is created.  If you use the single document workflow in particular then every time you use a new language pair Studio will add that pair to the default template.  The effect being that when you also use the standard project creation workflow with the default template you will end up with all the language pairs in you project.

So perhaps the answer is clean up your default template:

File -> Setup -> Project Templates -> Edit the default

And then perhaps learn how to work with Project Templates and create specific templates for the customers, domains or even languages you work with.  That way it's faster to create your projects, and you will avoid the sort of problem I think you're describing here.

I think this question needs some clarification.  Hopefully whoever asked this will see the post and be able to add some more context.

Unless the question simply relates to changing the project template to only use one language pair when the project is created.  If you use the single document workflow in particular then every time you use a new language pair Studio will add that pair to the default template.  The effect being that when you also use the standard project creation workflow with the default template you will end up with all the language pairs in you project.

So perhaps the answer is clean up your default template:

File -> Setup -> Project Templates -> Edit the default

And then perhaps learn how to work with Project Templates and create specific templates for the customers, domains or even languages you work with.  That way it's faster to create your projects, and you will avoid the sort of problem I think you're describing here.