Reminder: custom distros may already include another language than en-GB and its default installation language selected.

how does the existing installer work with that? does it grey out languages that are installed or?

Nothing special in 3.x.
The localise.xml file defines a default install language, thus it is already selected.
Nothing is modified when getting to the install language list if the user goes further there during installation. I would have to test again to find if the choice is given or not for defined default languages between both already present languages. (not on desktop now).

Are there still language distributions? I thought they had all stopped.

@wilsonge

Example of the French custom distro:
Joomla_394-Stable-Full_Package_French_v1.zip

The custom distro contains fr-FR in the pack

This is what we get when we install:

1. The installation uses the installation/localise.xml

<?xml version="1.0" encoding="utf-8"?>
<localise version="3.9" client="installation">
	<forceLang>fr-FR</forceLang>
	<helpurl></helpurl>
	<debug>0</debug>
	<sampledata>sample_data_fr.sql</sampledata>
	<params/>
</localise>

2. This will pre-select the installation language:

3. It pre-selects a sample data

4. When we get to installing languages, it does NOT propose fr-FR as possible install:
   (One can evidently install any other language)

5. fr-FR is proposed as default site and admin languages

in the joomla.sql we can find:
_extensions table:

[...]
(11, 0, 'com_languages', 'component', 'com_languages', '', 1, 1, 1, 1, '', '{"administrator":"fr-FR","site":"fr-FR"}', '', '', 0, '0000-00-00 00:00:00', 0, 0),
[...]
(600, 802, 'English (en-GB)', 'language', 'en-GB', '', 0, 1, 1, 1, '', '', '', '', 0, '0000-00-00 00:00:00', 0, 0),
(601, 802, 'English (en-GB)', 'language', 'en-GB', '', 1, 1, 1, 1, '', '', '', '', 0, '0000-00-00 00:00:00', 0, 0),
(602, 803, 'French (fr-FR)' , 'language', 'fr-FR', '', 0, 1, 0, 0, '', '', '', '', 0, '0000-00-00 00:00:00', 0, 0),
(603, 803, 'French (fr-FR)' , 'language', 'fr-FR', '', 1, 1, 0, 0, '', '', '', '', 0, '0000-00-00 00:00:00', 0, 0),
(700, 0, 'files_joomla', 'file', 'joomla', '', 0, 1, 1, 1, '', '', '', '', 0, '0000-00-00 00:00:00', 0, 0),
(802, 0, 'English (en-GB) Language Pack', 'package', 'pkg_en-GB', '', 0, 1, 1, 1, '', '', '', '', 0, '0000-00-00 00:00:00', 0, 0),
(803, 0, 'French_(fr-FR) Language Pack' , 'package', 'pkg_fr-FR', '', 0, 1, 1, 0, '', '', '', '', 0, '0000-00-00 00:00:00', 0, 0);
[...]

_languages table

INSERT INTO `#__languages` (`lang_id`, `lang_code`, `title`, `title_native`, `sef`, `image`, `description`, `metakey`, `metadesc`, `sitename`, `published`, `access`, `ordering`) VALUES
(1, 'en-GB', 'English (en-GB)', 'English (United Kingdom)', 'en', 'en_gb', '', '', '', '', 1, 1, 2),
(2, 'fr-FR', 'Français (fr-FR)', 'Français (France)'      , 'fr', 'fr_fr', '', '', '', '', 1, 1, 1);

_menu types

INSERT IGNORE INTO `#__menu_types` (`id`, `asset_id`, `menutype`, `title`, `description`, `client_id`) VALUES
(1, 0, 'mainmenu', 'Menu principal', 'Le menu principal du site', 0);

The _usergroups is also customised

INSERT INTO `#__usergroups` (`id`, `parent_id`, `lft`, `rgt`, `title`) VALUES
(1, 0, 1, 18, 'Public'),
(2, 1, 8, 15, 'Enregistré'),
(3, 2, 9, 14, 'Auteur'),
(4, 3, 10, 13, 'Rédacteur'),
(5, 4, 11, 12, 'Validateur'),
(6, 1, 4, 7, 'Gestionnaire'),
(7, 6, 5, 6, 'Administrateur'),
(8, 1, 16, 17, 'Super Utilisateur'),
(9, 1, 2, 3, 'Invité');

And as well the _viewlevels

INSERT INTO `#__viewlevels` (`id`, `title`, `ordering`, `rules`) VALUES
(1, 'Accès public', 0, '[1]'),
(2, 'Accès enregistré', 2, '[6,2,8]'),
(3, 'Accès spécial', 3, '[6,3,8]'),
(5, 'Accès invité', 1, '[9]'),
(6, 'Accès super utilisateur', 4, '[8]');

Duplicate #22907?

Yup. Closed that one in favour of this as we have the discussion about the localisation packs there. However for anyone looking here make sure you look at #22907 as there are some nice design prototypes

Closing this due to not receiving required information to determine if this is a bug or not. If you feel this still needs review, please open a new tracker entry with as much information as possible to ensure it can be reviewed properly

I am re-opening this - it should not be closed. Please Please stop closing issues without reading them. And please stop using a default text that has no relevance to the topic.