The Weather widget is English-only when you install Tiger. It can be translated as easily as it can be done for most any application.
The widget is located in /Library/Widgets. Change the permissions of the files so you can write to it (using File: Get Info). Open the widget's bundle (Control-click and Show Package Contents), and then copy the English.lproj to (for example) French.lproj. Then edit the file named localizedStrings.js with your favorite pure-text editor. When done, save the file, reset the permissions, and reload the widget.
Read the rest of the hint for the French translation instructions, including a caution about accented characters.
To translate in French, the localizedStrings.js file should look like this:
var localizedStrings = new Array;
localizedStrings['SUN'] = 'DIM';
localizedStrings['MON'] = 'LUN';
localizedStrings['TUE'] = 'MAR';
localizedStrings['WED'] = 'MER';
localizedStrings['THU'] = 'JEU';
localizedStrings['FRI'] = 'VEN';
localizedStrings['SAT'] = 'SAM';
localizedStrings['Include lows in 6-day forecast'] = 'Inclure les pr?©visions 6 jours';
localizedStrings['¬?F'] = '¬?F';
localizedStrings['¬?C'] = '¬?C';
localizedStrings['Degrees:'] = 'Degr?©s:';
localizedStrings['City, State or ZIP Code:'] = 'Ville ou code postal:';
localizedStrings['Validating'] = 'Validation';
localizedStrings['Done'] = 'Termin?©';
localizedStrings['No cities found'] = 'Ville introuvable';
localizedStrings['H: '] = 'H: ';
localizedStrings['L: '] = 'B: ';
localizedStrings['UseCelcius'] = '1';
localizedStrings['Try a more specific search'] = 'Essayer une recherche plus sp?©cifique';
Take note that accented characters are handled specially in this file. For instance, an "é" is représented by "?©". To find out which character representations were needed, I looked at other widgets, but I do not have a better source as for now.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050503200129790