This reminds me of the time when I removed en-US from windows, leaving just en-GB, and it blue screened. It's both surprising and irritating how many US-centric things are just assumed. (Don't even get me started on paper sizes...! ;) )
Europe uses the algo according to the ISO 8601 standard, where the week no.1 is the one with January 1st occuring between Monday and Thursday, inclusive. If the 1st happens on Friday or later within a week, it's considered a week no.52 or 53 of the previous year.
US does not use this scheme and [EDITED] I am not sure what method is applied there.
Some companies use week numbers in business talks, planning and scheduling, so be aware who is speaking about which weeks!