Creating a language programmatically looks straightforward and is not. This fails, silently at first and then confusingly:

$lang = new \IPS\Lang;
$lang->short = 'he';        // does nothing useful
$lang->title = 'עברית';
$lang->save();              // lang_short is left EMPTY

set_short() is a side effect, not a setter

Lang::set_short() calls setlocale(), stores the resulting locale data and, on Windows, the codepage. It never assigns lang_short. So the column stays empty, the getter returns NULL, and then languageInit() — which calls set_short( $this->short ) — throws a TypeError the moment anything touches the object.

The AdminCP avoids all of this because lang_short arrives through form values rather than through the property.

What actually works

$id = \IPS\Db::i()->insert( 'core_sys_lang', array(
    'lang_short'   => 'he_IL',
    'lang_title'   => 'עברית',
    'lang_isrtl'   => 1,
    'lang_enabled' => 1,
) );

// constructFromData, not load() - load() returns whatever is in the
// multiton cache, which for a row created this instant is nothing useful
$lang = \IPS\Lang::constructFromData(
    \IPS\Db::i()->select( '*', 'core_sys_lang', array( 'lang_id=?', $id ) )->first()
);

Why an empty locale is worse than it looks

• Right-to-left cannot be detected. Nothing can tell that a language is Arabic if there is no code to read, so an RTL community renders left-to-right and looks broken.
• The flag in the language chooser is derived from the locale, falling back to treating a bare language code as a country code. So ar shows the flag of Argentina next to العربية, and zh_Hans shows no flag at all because it is not a country. Always include a region: ar_SA, zh_CN, he_IL.

A new language has no strings at all

Creating a language does not copy the string table. It has zero rows in core_sys_lang_words and the suite falls back to the default language for anything missing. So translating a language means inserting rows, and "how complete is it" is a comparison against the default language rather than a count of empty fields.

That is also why a half-finished language is a working site rather than a broken one.

Related: addToStack() often returns a placeholder, not translated text, A module shows as module__myapp_thing on the administrator restrictions screen, A core/Notifications extension needs a language key named after the CLASS