Hello, I can help you out here.
Unfortunately, anything include in the plugin directory on updates is overwritten including the language files. There is not function feature around that.
I did find a solution if you would like to do a little coding in this Stack Exchange:
http://wordpress.stackexchange.com/questions/121463/possible-to-add-a-language-file-to-a-plugin-without-adding-po-mo-files-to-plug
The Main answer has a way to build a plugin and use that to override another plugins language files.
That is the only option I have for you besides after an updating uploading your language files back into the plugin.
Let me know if you have any questions.
Thanks