There are different scenarios:
Sometimes you have books with explicit defined languages for a word, a paragraph, a file or book wide.
You can add a additional language to the spellchecker. These will be used for all explicit signalized words, paragraphs and so on (e.g. like in your exampe).
Usually there is only one language defined in a book. For this cases I use user defined dictionaries (mostly two: one for foreign language words and one for special word constructs used in the actual book). The user defined dictionaries can set to an active / non active status so that you have all freedom to use is like you want.
The only thing what is missing is the possibility to deactivate one of the main dictionary in case you use more then one dictionary. I thought this was implemented years ago when the spellchecker was implemented but I can't find it again, maybe I remember wrong and had only ask for it
For this I use a little trick to work first with the foreign language as false positive and copy all correct identified words in a new user dictionary for a foreign language and switch then back to to major language including the new user dictionary as additional dictionary.
Take a look to the section Import word lists. This is very helpful to manage huge word lists. It have the possibility to add a language identifier to a list or a word too. You can copy containing words of a user dictionary to clipboard to create your own sets of useful combinations
