CSV files used for upload must be UTF-8 encoded. If you use a different encoding, umlauts and special characters are not displayed correctly in Webtrekk Analytics (and in exported data).
Example: You use an ANSI encoding instead of UTF-8, and your upload file for the German website contains the word "Schutzausrüstung". Your editor will usually display this word correctly, as it adjusts to the ANSI encoding of the CSV file.
If, for example, you import the CSV file into Excel and use the default encoding (usually "1252: Western European (Windows)"), "Schutzausrüstung" is displayed correctly.
However, if you set UTF-8 for the import (as it is the case with Webtrekk), the umlaut will be displayed incorrectly (it would appear like this also in Analytics).
Therefore, when creating CSV files, use UTF-8 encoding for the upload or convert differently encoded files to UTF-8 before uploading.