From The Document Foundation Wiki
- If your text file does not use the same decimal separator as your current linguistic context, you will need to specify the language of the imported file. For example, if you are using LibreOffice in French, and you want to import a text file using a dot as a decimal separator, you must specify the language of the imported file as "English US".
- The preceding case allows the problem to be resolved "globally" if your text file uses the same separator throughout. But it may be that certain columns use a dot and others a comma, for example. In this case, you must select each of the columns concerned and choose the format to use.