Last night I spent a couple of hours with Zend Framework and especially Zend_Form. I discovered, and now also reported as ZF-6175, a bug in Zend_Validate_Float when using a locale with a decimal point other than ".". There are unit tests but none that test Zend_Validate_Float under a different locale.
He points out that floats are locale-aware in PHP itself (code examples) and how the current Zend_Validate float package handled it. He has already proposed a fix for the problem and, until its changed in the framework, has provided the simplified code you can use.