My document has a lot of non-English interspersed which I want to highlight. Is there any way to do this?
If you formatted the Paragraphs by styles, then you can search the applied styles by name (what contains the language property ‘English’ or other language).
Use the Find and Replace feature with the Other options - paragraph styles.
If the character has the Language set, you can Find .*, with [x] Regular expressions, and after pressing the Format button choose the language in the Font tab, then Find All.
You can see the Language of a text in the status bar (bottom center).
Tested with LibreOffice 7.3.7.2 on Windows 10.0.
Writing multilingual text requires some preparation.
As always styles are your friends. Styles are tools to semantically markup your text (styles don’t describe the look, they annotate text according to author’s significance and intent; it is only in a secondary step that styles receive distinctive visual attributes). Usually, i.e. in mono-lingual text, styles advertise bulk narrative, headings, comments, notes, emphasis, importance, irony, understatement, trademarks, … You can also add the language.
Your text has a main language which will be set in Default Paragraph Style, Font tab, Language menu. Setting it in Default Paragraph Style will be automatically forwarded to all other styles, including Text Body the normal style for narrative.
Now if your text only includes foreign words here and there, as insertions in the middle of “ordinary” paragraphs", just create a Foreign Word character style with language set to None. It will be used for all words not in your preferred language. You can the add any highlighting to the style.
If your text contains full paragraphs in some other language, it is preferable to duplicate the “normal” paragraph styles, at first Text Body, with language set adequately so that check spelling is done properly. You can keep the idea of none-language Foreign Word to be able to insert primary language words in your secondary language paragraphs.
You can also create dedicated character styles for your various languages if you nedd to distinguish between English, French, Italian, … words but it becomes complicated to manage all this.