As of WPML 4.3, WPML uses MO files to deliver String Translation without loading the database. For this to work, WPML needs to be able to write to the languages folder whenever you translate strings.
If you are seeing a message that tells you that WPML cannot write MO files, 99% of the time it’s a file permission problem. WPML is attempting to write to the languages folder in WordPress, but that folder is not accessible.
If you are developing locally, it’s easy to change the write permission of folders in the WordPress directory. You don’t need to open all paths for writing. Navigate to your WordPress installation, and then make the wp-content/languages folder writable.
Most hosting companies don’t allow you to modify the write permission for folders. If you are using a shared host or a managed host, you will need to request help to open this directory for writing.
Please note that you are asking to open a very safe folder. This is the same folder where WordPress itself stores MO files that it downloads for you. WPML’s error message will give the full path of the folder that needs changing. Please copy/paste that path and ask your hosting company to make it writable, so that WPML can save its MO files.
If you are using a version of WPML older than WPML 4.3 and cannot get the languages folder to be writable, please open a ticket in our support forum.