I have found the culprit:

$matches = preg_split( "/([T]?[0-2]?[0-9]:[\:0-9]+[+\-]?[0-2]?[0-9\:]+|[a-z,A-Z]+|[0-9]+|[ ])/u", $parsevalue , -1, PREG_SPLIT_DELIM_CAPTURE | PREG_SPLIT_NO_EMPTY );

in includes/datavalues/SMW_DV_Time.php:202 (SMW 1.8.0.4).

For instance, with the date "8 décembre 1999" (december), $matches is:
Array
(

[0] => 8
[1] =>  
[2] => d
[3] => \xc3\xa9
[4] => cembre
[5] =>  
[6] => 1999

)

It should be something like:
Array
(

[0] => 8
[1] =>  
[2] => d\xc3\xa9cembre
[3] =>  
[4] => 1999

)

I replaced [a-z,A-Z] by [\p{L}] to match any Unicode letter character (see http://www.php.net/manual/en/regexp.reference.unicode.php) and it works fine.

I think it's a harmless change. Any opinion?

That looks good, thanks for reporting, investigating and providing a patch :) I will try it out now.

https://gerrit.wikimedia.org/r/#/c/57422/