Hi,
The geolocation plugin uses the IP address of the customer to set the current zone. It's useful when you have different prices / taxes / currencies for different zones / conutries.
But it won't help for the language.
You can select the orders for which you want to change the language in the orders listing, then click on the "actions" button at the top, select the action "update the values", select the column order_lang, select the mode "string" and enter in the input field the text en-GB and when you validate, all the selected orders will have their language changed to en-GB and thus the email notifications will be sent to the customers in English for these orders after this.