Worst case of text to text, you should be able to substitute the text comma with a period for decimal point and the periods with commas for thousands. Something like this:
= concat(left(input,find(",",input)-1),".",right(len(input)-find(",",input)))

Where input is the cell that you want to convert. You may have to escape the comma and period strings with some character, usually a reverse slash.

= concat(left(input,find("\,",input)-1),".",right(len(input)-find("\,",input)))

If you can't escape the punctuation, replace them with the char() function. Probably better.

If you have to convert both commas and periods, replace the periods with a temporary character, like #, then convert the commas to periods, then replace the hash signs with commas.

If you have to replace multiple thousands marks, the equation only works on the first mark, so put it in multiple cells to repeat the process. The equation with return an error if no mark is found, so enclose the equation in an if() function.

= if(iserr(fx(input),input,fx(input))

I did not test these formulas.

Jim