Why is the test Salesforce login in the wrong language?

I get this a lot as I switch between customers' environments who are in different languages. I believe the test Salesforce login in the wrong language is due to your locale on your Language & Time Zone settings on your profile. So, I signed into a Dutch customer's environment and my Locale is Dutch (Netherlands). Then, I go to sign into another environment where my Locale is English (United Kingdom) and I am greeted with Dutch. I think it is to do with the cookies that get stored locally, once you sign in the language should reset to your correct Locale. If you want to change your language so they are consistent across orgs, go to Your Name | My Settings | Personal -> Language & Timezone and change to the one you want.