I have not found a way to set the language in ubuntu to check the spelling, but not write the menus in that language.
My case is: my main language is dutch, and I want my system in dutch, with english as secondary language. Only those two languages for menus and terminal communication etc. But I would need spell check in english, dutch, french, hungarian and maybe german. This not only in certain programs like libreoffice, but system-wide (while I'm typing all of this, my screen's full of red).
Until now, the only way I have found to get spell check working in all of those languages is to install and order them all in language support (or however it is called in english), which makes a huge mess of all kinds of different languages mixed through one another.
Running Ubuntu 16.04 LTS
EDIT: Here is my language support arrangement for now
- Spell check
- All your languages
- English (US)
These seem fairly random to me, I have rebooted after having changed the language settings.