In Excel, If you want to format to format a date using weekday names or similar, the default format has an implied language. If you explicitly want to specify which language/culture you want to use in the format, you can use the following formula:
=TEXT(B2;"[$-sv-SE]DDD")
This will turn an Excel date in to the Swedish short name of that weekday, for example "Mån" for a date representing a monday.