There are at least two aspects to consider.
You didn’t mention your OS in the question. Packaging policy varies among OS’es.
Out-of-the-shelf LO is usually installed with only English USA (en_US). Depending on the OS, you must either install the specific English language pack (containing many variants) from the distribution repository or direct the installer to install the languages you need (notably to force installation of other English variants).
You can tell if a given language is installed if the language name has a small icon (usually depicting something like an A) at left of it in the drop-down menu.
Without the needed dictionaries and other language-specific modules like spell-checker, hyphenator, …, you can still type in the language but spellcheck, hyphenation, … will not be available.
Language for a paragraph or word is defined in the paragraph or character style. As with any attribute, you can override this setting with direct formatting. Direct formatting is “invisible” and takes precedence over the styles. Therefore, your styles may show English (UK) but your text will be in another language. You can tell the occurrence of such an override by putting the cursor in the text and reading the current language in the bottom status bar.
Your styles inherit the default setting in
Languages provided there is no override in the inheritance chain.
One common mistake is the use of Default Style for document content instead of Text Body. Default Style is the ancestor of all other styles and its settings are forwarded to all. It is intended only to set defaults, not to be used for text.
If you have modified Default Text to set a language there, this language will cascade to all other styles. To erase the modification and revert to the original “pass-thru” state, open Default Style definition, go to the Font tab and press
Reset is a misnomer: it only cancels the modifications you’ve made since you opened the dialog. It does not reset to factory-state.
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