OK I have just learnt something.

If I change the "generic.php" file in my default English folder, I can successfully change it to Japanese.
However, on the front end, whatever language I choose (in the bottom left language selector), the Japanese　text remains.

So it seem that the generic.php file in the English folder is being read/used for ANY language? That seems logical explanation to me anyway. Is that setting fixed?
If not, how can I make it use the Japanese generic.php file for Japanese, the English generic.php file for English, etc?

Thanks