Won't this mean that if a language has been set to anything other than public it will be changed on an update.

I have already considered that this access field can be changed. That's why I made a restriction:
ANDaccess= 0 to apply changes only in case when access is not set.

i think it would be better to have that change in new sql files.

Also since a user can easily change the language title, wouldn't be better to change all languages that have access level of '0'? (there is no access level with 0 id)

UPDATE `#__languages` SET `access` = 1 WHERE `access` = 0;

Even then, a issue that remains is if the user somehow deleted the view access level with the id 1 (Public). Probably that's a really wild scenario since for deleting the Public access level you need to have no content with Public access level.

Isn't that a known issue for updated sites and you actually should have a post-install message about it?

Setting it to 1 isn't the correct thing to do. The access level with the id 1 can be anything. It just happens to be "public" on most sites because it's preinstalled this way, but you can change that.
That's why we decided back then to not have an update SQL but a postinstall message.

See https://github.com/joomla/joomla-cms/blob/staging/administrator/components/com_admin/postinstall/languageaccess340.php for the relevant postinstall code.

yeah, @Bakual is right IMO.

I knew there was a message just couldn't find it. Thanks

I'm closing this as a known issue which will not be fixed.
See also https://docs.joomla.org/J3.x:Infinite_error_loop_on_multilanguage_sites

also, we prvented an error in the languagefilter. see #6194