Right-click the field and select "Change field..." (or whatever the item is called in your language of Word).
In my case, the field is BIBLIOGRAPHY \l 1053. 1053 is the Microsoft Locale ID (LCID) for Sweden. I just changed it to BIBLIOGRAPHY \l 2057, where 2057 is the LCID for English - United Kingdom.
Then the bibliography changed language, so there is no more "Fifth upplagan" instead of "Fifth edition"!
Reference:
