You change language in webmail in one setting in the ”Settings” page. The pictures below show how to do it when the language is Swedish. Regardless of the language, the controls are located in the same place, so you should hopefully with these instructions be able to change from any language to the language of your choice.
Click on Inställningar, the second icon from the right in the top row in the window.
Click on Användargränssnitt, the topmost item in the left menu.
Click on the pop-up menu with a series of names of languages in it.
Select the language of your choice.
Save your new setting by clicking on Spara (or whatever Save might be in the old language you are swapping from.