regression: show language selection option not auto checked (after fixing show/hide backwards)

Follow up for #1853720: Hide language selection option is backwards

Problem/Motivation

It used to be that by default the hide language checkbox was checked. And when on the content language settings config page, with content translation enabled, when the translatable checkbox was checked, that the hide language checkbox was auto unchecked.
The issue to fix the hide checkbox being backwards must have changed the behavior.
Now, the show language selection checkbox is unchecked by default (ok.) but when translation is enabled, the show language selection checkbox should be toggled to be checked. (Not doing this leads to confusion.)

Proposed resolution

Fix the show language selection checkbox to be enabled when translation is enabled on admin/config/regional/content-language

Remaining tasks

• look at related issue to find where in the code to fix.
• create initial patch

User interface changes

API changes

No api changes.