I will strongly suggest to the developers of Arastta not to change file names just to be different, they should keep original opencart file names as far as possible to have complete software compatibility. I don't know who's great idea it was to change language file name to "en-GB" in place of "english" by doing this irrelevant and senseless change the developer is preventing users from using their preferred extensions or modules as the associated language file is rendered useless and the new installed file does not load the contents into "en-GB" file so why this change was made is a mystery to me.