Multi-lingual languages are quite hard to manage. Success requires the use of consistent “semantic” styling. In this approach, style names don’t describe the visual aspect of text but tag text as to its significance.
You have to design two sets of paragraph and character styles.
The paragraph styles describe the general aspect of paragraph and you set the language in Font tab, therefore two sets. You can use the built-in styles like Body Text and its children for your primary language and create a Body Text, Klingon for the other one (here Klingon 
), plus the children.
You also need character styles for words or sentences in a different language than the the rest of the paragraph. If you only need to change language, just define Klingon, English, … to temporarily swith to the designated language. It is also possible to apply several character styles (e.g. to add Emphasis or Strong Emphasis) but in the present implementation I’d rather recommend to again create a full second set because managing edits when several character styles are applied is quite difficult if you want to avoid to mess up everything.
If headings may be in several languages, you must switch language with a character style because you can have only one paragraph style (Heading n) per level in the chapter numbering engine. However, if localised headings belong in separate parts of the book with their own numbering, you can design a seconf Heading-like hierarchy. But, then, you must also master list styles.
IMPORTANT: what is said above is valid ONLY if your document is saved .odt. Any other alien format will lead to failure because they have no character style feature. ODF encoding will be replaced by direct-formatting and you won’t be able to tune your document appearance.
PS: when asking here, always mention OS name, LO version and save format.
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