The spelchecker is a very usefull feature as well as the program setting to change the User Interface Language.
The spellchecker is directly dependant on the general language setting in program settings. There is no option to set the application user interface to a default language (English) and the spellchecker to my mother tongue or any other language.
I just think that the distinction should be made. A user interface, translated into my mother tongue, isn't always so easy to understand as the original and therefore I prefer to work in a standard language for all applications on my device but do my own scripts in my mother tongue.
To support this further. I sometimes write scripts in other languages as well and would like to change the spellchecker but not necessarily the user interface.