Configuration entities created after / in installation on sites get created in the site default language. That is because we can rightly assume you enter data in your own site language initially. That is absolutely fine for almost all things. You'd create views and enter their labels, empty text, etc. in your language. And so on. However for languages, you pick them from a predefined list of English language names in 95% (estimated). We save them with their English label. Then translate from there. So the assumption about config entities does not stand.
1. Force language config entity creation to always save in langcode: en.
2. Change the UI label in custom language creation to 'Language name in English' to make this clear if custom languages are created (for 5% of uses - estimated).
PASSED: [[SimpleTest]]: [MySQL] 56,958 pass(es).
PASSED: [[SimpleTest]]: [MySQL] 56,671 pass(es).
FAILED: [[SimpleTest]]: [MySQL] 56,461 pass(es), 2 fail(s), and 0 exception(s).
FAILED: [[SimpleTest]]: [MySQL] 56,496 pass(es), 1 fail(s), and 0 exception(s).