Not silly, just not all that easy to do.  Changed to feature request.

int strtotime ( string time [, int now])

is the actual function. As it gets even a complex input from the string "time", I think it's difficult to emulate these functionality in a localized fashion. However, much of the international users are concerned about the output of the function. That could be modified without such a difficulty. Here's a proposal

int strtotime ( string time [, int now] [, string ccode])

where the optional string "ccode" handles the localization request. Giving IT the result will be in Italian, EN or US for plain English, DE for German, FR for French and so on.

So when requesting a date string, outputting in French, the software only has to know the country specific position of the substrings week, month, day, year and the name of weeks and months in that language.
This could not work for other calendar dates, as chinese.
Submitting 10+ languages could be a matter of hours work, and backward compatibility should not be an issue, as the parameter "ccode" is optional. This could be a little step for the PHP source code, but a big step for the PHP community worldwide.

I think behaving on locale could trigger compatibility problems.

Bogus.

I want to propose simple solution for this problem: http://www.phpclasses.org/package/6551-PHP-Manipulating-with-localized-date-time-strings.html