In CRM 2015 Online Update 1 when the user clicks "Export to Excel - Open in Excel Online", the contents of the CRM records view is not shown in the same format as specified in the CRM user's format settings. Instead the Excel Online window always uses the US format. Changing the user settings in Office 365 user profile doesn't seem to have any effect.
While viewing this data in Excel Online the wrong formats are mainly an inconvenience for most users. However, if the user makes modifications to any cell values and tries to import the changed data or new rows back to CRM, this will cause incorrect data to be entered to CRM, or alternatively import error entries as CRM will interpret the values in the format set for the CRM user, not the ones that Excel Online has used for constructing the import file behind the scenes.
For example, if I have an entity with a date field and a record that has June 1st as the value, this will show up as "1.6.2015" in the CRM UI, based on the Finnish format settings. When I open the same records in Excel Online, the value will read as "6/1/2015" based on the US settings that are applied there. Now, if I want to set that date to today's date, which is June 14th, I'll need to change the value to "6/14/2015", following the US format. When I then try to save this data back to CRM, this record will not be imported, as CRM will interpret the value as "6.14.2015", which is obviously not valid since there isn't a month to match the number 14.