Hello,
I might think about this in the future, but I believe it is easy to work around. Dictionary files are plain text files. If you look at en_US.dic in the Dictionaries folder of the EmEditor install path, you will see the list of words. You can simply combine two dictionaries to create a new language. You will also need .aff file. I believe you can use the same file for two languages.
Another workaround is you can append either one dictionary to “UserDic.bin” file located in %appdata%\Emurasoft\EmEditor\Spell. “UserDic.bin” is a user dictionary. When you open “UserDic.bin“, you will need to specify UTF-16LE without signature.