Only issue is that when I edit the english.php file and upload the file the product page stops working. I've edited it in notepad so how should I edit these php files?
That indicates a corrupt edit or transfer
Notepad is an acceptable "plain Text" editor
Are you sure that you are including the pair of single quotes
Code:
define('TEXT_PRODUCT_WEIGHT_UNIT','lbs');
Try downloading the one from the server that breaks the site
Open it in notepad and chack gor any leading lines or spaces at the top and trailing lines or spaces at the bottom
If found determine what process is adding/corrupting the file
Most likely FTP