What is the risk of changing the "Default locale" option in languages? Since it's used to get translations with getText(), Will they still be available if I change this option? For example, if I saved a string translation for french and the default locale was fr_FR and now I'm changing it to fr_CA, will the translations still be available?
Welcome to WPML support. I'd be assisting you further on this issue.
Default locale helps to retrieve the translations from the .po/.mo files for the plugins/themes as well as for WordPress core, so if you change the default_locale option you should also modify your .po/.mo files accordingly available in wp-content/languages folder.
Also, you will need to manually download the .po/.mo files for plugins/themes and add it to the folder with current locale codes, the download translations from wp-admin/update-core.php will not be effective, it will search for default locals defined.
The string translated under string translation will not have any effect due to this change.