It's most likely a caching issue (your browser is maybe still using an older version of your gallery's 'config.xml' file prior to the change to backButtonText having been made).
try completely clearing your browser's cache before reloading your web page to see if this resolves the problem.
Otherwise, try viewing your web page in a different browser (one that has never been used to view the web page before) to see if the backButtonText is as expected.
If the problem persists, then perhaps your web server has some server-side caching enabled which is still serving the older 'config.xml' files prior to you uploading the new version. If this is the case, then please contact your web host and ask if they can disable server-side caching on your hosting account.
I hope this helps (as your configuration looks OK).
However, if you continue to experience difficulties, then please post back with the URL to your web page so that I can see the problem for myself live.
Once I'm able to see the problem live on your web server, I should hopefully be able to determine the exact cause of the problem and propose a solution.