How do I explicitly add specific languages to "Tools > Language > All Text" in LibreOffice Writer?
Download the desired language extension (*.oxt file) from here
After download right-click on this file and select open with
libreofficewriter and open "Tools > Language > All Text" and select "more"
Now "Options- Language setting" will open. Here select desired language in "Locale Setting" tab and press apply. You are done.
AFAIK, there is no way to make the UI dependent on custom configuration (I mean users may load additional language packs, every user having different need from another one).
Since your question is about All Text, this means you intend to write a document in one of the cited languages. Even if you have a few paragraphs written differently, you basically end with a document in a dedicated language.
My best advice is to create as many custom templates as you have main target languages: one for en_UK, one for es_SP, one for es_AR. The main difference between these template is in paragraph style Default Style Font tab where you set the language once for all.
You can also customise the paragraph styles if cultural typographical rules differ with the languages. For example, in en_US, paragraphs have usually no first line indent while in other languages it is customary to have a first line indent. This difference can be recorded in Text Body.
When you create a new document, you start from the appropriate template.
The solution seems to be adding templates.