Utilize site languages in th language selections

Problem/Motivation

Currently at least the CKEditor5 plugin to translate content to other languages require that you write the language name manually each time you translate something. This is a bit error prone and hard to use when you are doing a lot of translations. It can easily lead to typos etc.

I would presume that usually people are translating content from the selection of the site languages. Could we offer a list of site languages by default as selection and for example make the default language the default option? There could be an option to enter a language manually also but I think usually people are inputting a limited number of languages.

Steps to reproduce

1. Enable the CKEditor5 plugin for OpenAI.
2. Translate some text with CKEditor5 and open the language dropdown UI.

Proposed resolution

I would prefer that the language list would use the site's language list from admin/config/regional/language as the source for the translation.

Remaining tasks

TBD.

User interface changes

TBD.

API changes

TBD.

Data model changes

TBD.