There's my fork for Elgg 3.3 https://github.com/RiverVanRain/language_selector/tree/3.3

manually uploading the language files to my site.

BTW, you don't need to do this. All translated files will be available with the next Elgg release.

You can try this plugin also https://github.com/ColdTrick/translation_editor

Will you be updating Language Selector to Elgg 4? I'm already running 4.0.1. Will this mean any logged out user visiting the site will see the Gaelic version automatically or do they have change a setting?

The subject of the community I'm creating is Gaelic language and culture, so ideally I'd like minimise English on the site as much as possible.

I guessed the translated files would be added to the next release, I'm not always sure of the context of the strings I'm translating, so I like to have a live view as I'm working.

Thanks for that, I'll definitely give Translation Editor a try.

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 ;)