That option to change the text file properties will be available if you right click the sheet which you dragged in the canvas after connecting to the data and the option would be 'Text File properties', try changing the setting to english.
If this does not solve the problem, please upload the file with some sample data so that we can look further into this problem.
If this helps you resolve the problem mark this as correct and helpful.
Shekar's reply should work but in a rare case, There must be blank spaces present just before or just after the numbers in the fields. Hence, it might show numbers when the type is string and NULL when converted into decimal.
So, in that case try this
Create calculated field:
and use this as your field
Pleas attach a sample workbook in .twbx in case this doesn't work
Hope this helps!
If this reply helps, mark it as helpful/correct