For anyone reading this in the future. If you get the following error and the .csv file has a red X as its icon in Godot then your csv file uses a delimiter other than a comma. Your delimiter must be a comma
editor/import/resource_importer_csv_translation.cpp:95 - Condition "line.size() <= 1" is true. Returning: ERR_PARSE_ERROR
Error importing 'res://Locale/Localization.csv'.