I have followed these instructions to the letter:
https://docs.microsoft.com/en-us/sql/ssdt/download-sql-server-data-tools-ssdt?view=sql-server-2017#ssdt-for-vs-2017-standalone-installer
and this still keeps failing with the following error in the logfile:
Initializing string variable 'SystemLocaleMismatch' to value 'Setup detected mismatch between the languages of the installer and the OS. Use the English-language version installer, or install the language pack and change the format and system locales through the regional settings in the control panel.'