You can add url rewrites for them.
Go to Catalog->Url Rewrite Management and add a new url rewrite.
Select Custom
for Create URL Rewrite:
.
Then fill in the rest of the fields
Store : English
ID Path: Fill in a unique string here
Request Path: magasins
Target Path: stores
Redirect : Permanent (301)
Description: Not important. You can leave it blank
Save.
Then do the same thing again for the other way around.
Store : French
ID Path: Fill in a unique string here
Request Path: stores
Target Path: magasins
Redirect : Permanent (301)
Description: Not important. You can leave it blank
Save.
This should be just fine if you have only a few pages in this case. If you have a lot of them it becomes harder to manage.
In this case I recommend you this extension.
It allows you to do the same thing, but from a single screen. You just have to fill in the url keys for each page in each language and it will create the url rewrites on a single click.
I'm the extension developer, but the extension is free and I get not financial benefits from it. So this should not be considered as spam.