I think I correctly set up my override structure, and things appear to be coming together... (I think)... with the exception of the language files, which will not override the default files.

I created my custom directories as follows:
includes/languages/english/'custom' and includes/languages/'custom'

My first clue that the override was not working properly, was the fact that the sentence " You may want to add some instructions for using the shopping cart here. (defined in includes/languages/english/shopping_cart.php)" keeps appearing at the top of the page. This sentence is in the default file shopping_cart.php, and not in any of my custom files. Thus, the default files are still controlling things.

I put the meta_tag.php files in both custom folders. I have checked my spelling, and case sensitivity issues. I have checked my admin setttings.

Have I missed something that prevents my custom files from overriding the default files for the languages section?

Any insight would be appreciated.