TYPO3 Core

To clarify: Irish and Jamaican is not in the list of supported languages:

• docs: https://docs.typo3.org/m/typo3/reference-coreapi/master/en-us/ApiOverview/Internationalization/Languages.html
• authoritative list in core: https://github.com/TYPO3/typo3/blob/master/typo3/sysext/core/Classes/Localization/Locales.php ($languages)

So, this is not a bug. I am changing it to feature request.

Apart from that, I don't know if it makes sense to add support if we have no one to translate ...

So, this is actually a little misleading in the Introduction package: There, a page /content-examples/and-more/any-language-any-character is created with the following content:

TYPO3 Speaks Your Language
TYPO3 supports all languages, characters and encoding out-of-the-box. All frontend, backend and database interactions run Unicode (UTF-8) by default.

Bulgarian: Жълтата дюля беше щастлива, че пухът, който цъфна, замръзна като гьон.
Czech: Příliš žluťoučký kůň úpěl ďábelské kódy.
Dutch: Pa's wijze lynx bezag vroom het fikse aquaduct.
English: The quick brown fox jumps over the lazy dog.
Esperanto: Eĥoŝanĝo ĉiuĵaŭde.
French: Les naïfs ægithales hâtifs pondant à Noël où il gèle sont sûrs d'être déçus et de voir leurs drôles d'œufs abîmés.
German: Falsches Üben von Xylophonmusik quält jeden größeren Zwerg. (1)
German: Im finsteren Jagdschloß am offenen Felsquellwasser patzte der affig-flatterhafte kauzig-höf‌liche Bäcker über seinem versifften kniffligen C-Xylophon. (2)
Greek (monotonic): ξεσκεπάζω την ψυχοφθόρα βδελυγμία
Greek (polytonic): ξεσκεπάζω τὴν ψυχοφθόρα βδελυγμία
Hebrew:זה כיף סתם לשמוע איך תנצח קרפד עץ טוב בגן.
Hungarian: Árvíztűrő tükörfúrógép.
Icelandic: Sævör grét áðan því úlpan var ónýt.
Irish: "An ḃfuil do ċroí ag bualaḋ ó ḟaitíos an ġrá a ṁeall lena ṗóg éada ó ṡlí do leasa ṫú?" "D'ḟuascail Íosa Úrṁac na hÓiġe Beannaiṫe pór Éava agus Áḋaiṁ." 
Jamaican: Chruu, a kwik di kwik brong fox a jomp huova di liezi daag de, yu no siit?
Japanese (Hiragana):
いろはにほへど　ちりぬるを
わがよたれぞ　つねならむ
うゐのおくやま　けふこえて
あさきゆめみじ　ゑひもせず (4)
Polish: Pchnąć w tę łódź jeża lub ośm skrzyń fig.
Portuguese: O próximo vôo à noite sobre o Atlântico, põe freqüentemente o único médico. (3)
Sami (Northern): Vuol Ruoŧa geđggiid leat máŋga luosa ja čuovžža.
Slovak: Starý kôň na hŕbe kníh žuje tíško povädnuté ruže, na stĺpe sa ďateľ učí kvákať novú ódu o živote.
Spanish: El pingüino Wenceslao hizo kilómetros bajo exhaustiva lluvia y frío, añoraba a su querido cachorro.
Swedish: Flygande bäckasiner söka strax hwila på mjuka tuvor.
Russian: Съешь же ещё этих мягких французских булок да выпей чаю.
Russian: В чащах юга жил-был цитрус? Да, но фальшивый экземпляр! ёъ.

After further considerations, I found that there is still the need to translate to existing, already supported languages:

Adding further languages - unless there are people ready to translate - is, I think counterproductive.

I think I went a little overboard here and would like to close this issue.