Go to your Control Panel, then click on the Regional Settings option. Then, search for a function that lets you change the locale for non-Unicode programs. Don't forget to change your float symbol to a point by opening a window with a button on the first tab of the Region window, too, otherwise UTAU will hate otos.
Then, re-generate Defoko's voicebank by downloading the latest ZIP version of UTAU and extracting only the uta folder in your voice folder, then running mkdefoko.exe there. As for Teto, you only have to re-extract it.