Both '.' and ',' for decimal separator in LibreOffice Calc?

You can set the language (and thus, the decimal separator) even per cell (or per document, see below). Thus, if your language is set to French, with , as decimal separator, you can still use . as decimal separator (and accept documents respectively) if they have a proper language setting.

You can check this yourself: just modify the cell's language setting by using the cell formatting features (context menu or Menu Format -> Cells...). Thus, you can even mix different decimal separators in the same document.

Here's a small example:

My localisation is set to German, so the comma is the default decimal separator. A2 and C2 contain both the value 2.5. A2 was formatted with en-us language settings (has to be done before entering the value!), thus the value is recognized as number, and the sum in A3 is calculated correctly. C2 has no special language format, thus no number recognition, wring sum.

EDIT:

To set the language for a whole document, the easiest way is to modify the default cell style. To do so, open Menu View -> Styles or hit F11. Right-click on Default, select Modify, and select the Numbers tab. There you can set the default cell language setting.

After modifying the default cell style, you may have to restore the numeric values, e.g. by doing a regex-based search and replace (search for ^.*$, replace with & - see https://superuser.com/a/394170/84724 )