(minor) Fieldtypes (Array and similar): inconsistencies in KEYS and LABELS leading to translation differences

[ebeauchamps]

Bug Description

Related to commit ac18e9c
Related to issue #2515

The above commit has introduced the possibility to translate the "Optional" key for the Label heading of the ARRAY fieldtype:

This is now OK.

I was expecting to find the same behaviour in similar fieldtypes, but I am not. I am even still getting English strings.
CHECKBOXES:

I get "KEY (VALUE)" instead of "CLE"?
I am missing the (optional) string, too.
And as you can perhaps see the sentence above the grid of KEYS and VALUES isn't exactly the same (in English as in French)

BUTTON GROUP, RADIO, SELECT, :
Missing the (Optional) string: I only get KEY and LABEL (Clé and Etiquette in French)
(Optional) should be added.

How to Reproduce

Add an ARRAY fieldtype and similar fieldtypes in a fieldset.

Statamic version: 3.0.39

PHP version: 7.4.2

Install method (choose one):

• Fresh install from statamic/statamic