As indicated in a previous post, you can change your computer's locale settings to US. But this will change all your other program's locales, which sucks.
To fix this broken behaviour of SISAR, you can also modify the file \Shared\EPOC32\Tools\javalib\supportedlanguages.xml to make your locale look like US-English only to Symbian SDK applications.
Find the entry matching your locale settings (country and language) in that file, then change tags <TLanguage> and <opl-const> to match the tags in the en-US section. As an example, here is my configuration to make Symbian SDK tools work with my fr-FR locale :
the bits that are commented out are the original settings.
<locale language="fr" country="FR">
<!-- <TLanguage index="02">ELangFrench</TLanguage> -->
<!-- <opl-const>KLangFrench%</opl-const> -->