Users of the current Beta 2 version of ClassicPress are finding that it does not use the WordPress list of languages.
CP has installed their own list, which only supports the 11 most popular languages. If yours is not on the list, you are out of luck.
To work around this issue, a plugin has been created on GitHub as a temporary fix. It allows you to access the WordPress list of languages to find yours.
Most users would have no idea what GitHub is, yet alone what it’s for. But luckily there are clear instructions on how to download and install the plugin.
If you are still intimidated by going to a geeky place like GitHub, then use this direct link to get the zip file. Then upload it and activate it like any other plugin. You will then be able to set your missing language.
The logical thing to do would have been to pull the error and release a Beta 3 update. It should not be up to individual users to patch errors themselves.