Just like many coders, I work with different languages at the same time. For example, there might be 2 opened files at the same time:
``` translations_en.php translations_XX.php ```
Obviously, they will contain text in different languages. So if my spellcheck is set to EN, I will see all the words marked as mistakes right after switching to translations_XX.php, and vice versa. What's more, that file translations_XX.php usually contains extra text in English (especially true for comments, definitions, variable names, etc._, so even if I manually set spellcheck to XX language every time after switching to this file, many parts of text will be marked as incorrect anyways.
This could be easily solved by having an option to use 2 languages at the same time. I'm not sure how easy/difficult it is to implement, but such option exists in Claws Mail and it's amazing - if you set 2nd dictionary, spellcheck will only mark words as incorrect if they dion't math both of dictionaries.
Hope I explained it clearly enough...