In Writer, you have several style layers, the shallower overriding the deeper.
The deepest layer has no name and is rather virtual. Tools>Options, various tabs define basic defaults used by “primary” styles and the application in general.
Then come paragraph styles which can be organised hierarchically so that some of them can just override attributes in their “father” definition. As their name implies, paragraph styles define the properties of whole paragraphs.
In your case, your “ordinary” paragraphs will be written in English and this language is set with the Language drop-down menu in the Font tab.
Above paragraph, you have character styles which are used to format/customise a sequence of characters inside a paragraph. Character styles locally modify attributes of the paragraph with the exception of those meaningful only for a paragraph (such as margins, spacing, outline, …).
You’ll define at least one such style per language used in your document. The Font tab controls the font face (don’t change font size unless you have a real good reason for it) and the language. Correctly setting the latter avoids false warnings by erroneously using the English dictionary in spell checking other languages.
The last layer is called direct formatting. In any elaborate document, avoid it absolutely. It is not controlled by styles and creeps invisibly in your text. When a change of style is not reflected, suspect some “direct formatting” plays its nasty trick on your back. Sometimes you can’t even guess what was applied since you have no feedback!
To summarise, you’ll have to play with character and paragraph styles.
There’s one thing you can’t avoid: selecting the proper character style is a manual process. Consider having the style side-panel (character style view) always visible so that you can quickly double-click on a name to switch to it.
You can also define a keyboard shortcut for your most common styles so that you keep your hands on the keyboard instead of alternating between mouse and keyboard.
The most annoying inconvenience is switching between keyboard layouts. There is no really “comfortable” solution to it. Sometimes, I plug several physical keyboards on the computer (simple USB keyboards are cheap nowadays) with different key layouts so that I have visual hints. If you don’t find “foreign” keyboards, just glue stickers with adhesive tape. Eventually, you can customise key combinations (Alt, Meta, Menu, …) to quickly “jump” to a layout instead of cycling. Quite easy to do under Linux or MacOS (no experience on Windows).
The practical solution to your question is learning styles and some sort of procedure for fiddling with the keyboards (or layouts).
