A quick look in the code, this looks like a filesystem issue.

• Did the dataroot folder exist you provided in the installation steps?
• Are the permissions for that folder set correctly?
• What PHP version are you using?

If you still have problems could you maybe log the issue in GitHub https://github.com/Elgg/Elgg/issues that way it's easier for us (the core developers) to help you.

Will you be updating Language Selector to Elgg 4?

Unfortunately, this plugin will not work on the Elgg 4. 

Will this mean any logged out user visiting the site will see the Gaelic version automatically or do they have change a setting?

This plugin allows to select the language on the site for visitors.

For logged in users, the choice of language is already in the users' settings if you've enabled the languages in the site settings.

Also, you can set your custom language as default in the Site settings page.

Elgg will try to detect the language of logged out users based on their browser preference. So if you have set your browser to Scottish it should display the site in Scottish.

PS: thanks for contributing with the translations. In 4.0.2 you can see the fruits of your labor ;)

Free:

https://github.com/nlybe/Elgg-roles_ui

https://github.com/hypeJunction/Elgg3-hypeModerator

Paid:

https://es.wzm.me/products/view/1684/elgg-theme

All for Elgg 3 only.