One way to make iWeb sites in another language is to change the system language completely. Another way is to Control-click the iWeb application and select Get Info from the contextual menu. Then unfold the Languages tab, and remove all languages but the one you want to create the site in. This way you force iWeb to use that language.

Note however that sites already created in another language will get errors for missing files from the old language templates. This means you have to rebuild the site from scratch. Just create another site next to the old one, and copy and paste as much as possible (everything except dynamic content, etc. which contains language-specific stuff).

[robg adds: Rather than removing the languages, just remove the checkbox next to all languages other than the one you want to use. This makes it simple to re-enable the other languages, and still has the desired effect.]