In my writings, I use multiple languages and styles. I would like to have more than just the English dictionary to rely upon.
Run the install program again, then select Custom install instead of Full install, then select all languages that you want dictionaries for.
Go to http://extensions.libreoffice.org/extension-center, find necessary dictionaries and install them.
Also, read this tutorial: [Tutorial] Spell check and Language configuration (View topic) • Apache OpenOffice Community Forum
And do not forget to set the language for your paragraphs in Font tab, drop-down menu Language in the paragraph style definition dialog. For correct spell check, it is necessary to declare the language in which a paragraph is written. This means you need one (at least) paragraph style per language.
Exceptions in paragraphs (i.e. words in a different language) are handled through character styles to divert spell check to another dictionary.
See a longer explanation in my answer to this question.
Defining separate styles for different languages is an overkill, IMHO, as the text language changes automatically when you switch your input language.
Yes if you write in a single language and this language is the same as the OS one (the most frequent case among LO users). No if you are writing multi-lingual documents where each run of text must be checked against the correct dictionary. In multi-lingual documents, I include technical ones, where math, equations, … must be “escaped”.
In Tools → Options, Languages Settings → Languages, you already play with 3 languages: UI, locale and default for document, not counting Asian and CTL