There is no answer because it varies even inside a country.
However, this source seems to think it's defined by the ISO standard as the first week that contains a thursday. However the above source says that the ISO standard does not list a specification for 'first week of the month'
More than 300 chars - sorry!
The ISO 8601 standard defines the first week of the YEAR as:
calendar week number
ordinal number which identifies a calendar week within its calendar year according to the rule that the first
calendar week of a year is that one which includes the first Thursday of that year and that the last calendar
week of a calendar year is the week immediately preceding the first calendar week of the next calendar year
There is nothing in ISO 8601 about the first week of the month because, as has been noted, the definition varies by country (and even by company, for example). Oh, and the standard says that weeks start on Monday.