I've a doubt with lowercase, in v1.6.2 archive are UCfirst, but if you're sure, ignore it.

You're right, I think I made a mistake there.
Funny thing is that 'english' is lowercase and the rest is UCfirst.

I just remember me having troubles with those language files after last update

After renamed French folder to french IP say:

An Error Was Encountered

Unable to load the requested language file: language/French/ip_lang.php

UPDATE `ip_settings` SET
`setting_id` = '19',
`setting_key` = 'default_language',
`setting_value` = 'french'
WHERE `setting_id` = '19';

UPDATE `ip_users` SET
`user_id` = '1',
...
`user_language` = 'french',
...
WHERE `user_id` = '1';

#0 application/third_party/MX/Lang.php(66): CI_Lang->load()
#1 application/helpers/trans_helper.php(81): MX_Lang->load()
#2 application/core/Base_Controller.php(78): set_language()
#3 application/core/User_Controller.php(28): Base_Controller->__construct()
#4 application/core/Admin_Controller.php(21): User_Controller->__construct()
#5 application/modules/invoices/controllers/Invoices.php(24): Admin_Controller->__construct()
#6 vendor/codeigniter/framework/system/core/CodeIgniter.php(519): Invoices->__construct()
#7 index.php(319): require_once('...')
#8 {main}

$e = new \Exception;dd($e->getTraceAsString());exit;

It solved by change

    $new_language = ($language == 'system' ? $default_lang : $language);

To

    $new_language = strtolower($language == 'system' ? $default_lang : $language);

in application/helpers/trans_helper.php Line 75

But maybe need more investigation and verification if would be set the lowercase for languages folders. (settings page, users languages ...)

After little verification (and set language(s) to French in db):

Setting page lose system lang and for users/form/1 returned to System lang because (IMO) get_available_languages() in application/helpers/trans_helper.php Line 95 return french but when choose French in language selector and save, All it's ok (and in db it changed French to french 👍)

Notes:

IMO just apply strtolower with setup (updater) on all langs in db become transparent for users.

@nielsdrost7

Understand why after changed French to french by hand in db get an error Unable to load the requested language file.

Because IP get user lang in $_SESSION. Need to logout & login to apply new lang parameter.

I just remember me having troubles with those language files after last update

I checked with change (in admin) to an other lang Spanish and work fine, the bug will probably arrive when after update (langs lowercase) but in db no ;)

Maybe only need update languages in db for the old IP to solve the Red Screen Of Death if next version have folders language lowercase. For this see PR #1232

Fun fact: No problem with folder case (if it same name in db) only need to disconnect & reconnect.