fix: clarity issue with ambiguous string `SET` (#1169)

## Summary

* **What is the goal of this PR?** (e.g., Implements the new feature for
file uploading.)

Fixes a clarity issue regarding the translation string `STR_SET`. The
issue lies in the fact that the english word can have different
meanings.

The only time the string is used is in the language selectio screen,
where it has the meaning of _selected_. (As in _The language has been
**set** to French_).

Another meaning can be _configured_. (As in _The KOReader username has
been __set__). This is the meaning many of the translations have taken.
The reason that the string is right above `STR_NOT_SET` (which is meant
as _not configured_).

With this PR I propose to explicitly use the term "_Selected_". There
are two good reasons for this:
+ it removes the confusion and the misleading translations
+ it is consistent with the button label `Select`, communicating the
link between the two (the row will be marked `Selected` if you press the
buttpn `Select`. Much clearer than now)

* **What changes are included?**

Removed the unused strings and added translations for the new string
`STR_SELECTED` for the languages I know.

tagging the translators for feedback:
fr: @Spigaw @CaptainFrito
de: @DavidOrtmann
cs: @brbla
pt: @yagofarias
it: @andreaturchet @fargolinux
ru: @madebykir @mrtnvgr
es: @yeyeto2788 @Skrzakk @pablohc
sv: @dawiik
ca: @angeldenom
uj: @mirus-ua
be: @dexif

## Additional Context

* Add any other information that might be helpful for the reviewer
(e.g., performance implications, potential risks,
specific areas to focus on).

the Issue was introduced in #1020. Previously, if a language was
selected it was marked with `[ON]` (`STR_ON_MARKER`). I considered
reverting it back to that, but the solution I described above seemed
superior.

---

### AI Usage

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Did you use AI tools to help write this code? _**< NO >**_

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Co-authored-by: Егор Мартынов <martynovegorOF@yandex.ru>
Co-authored-by: Mirus <mirusim@gmail.com>