Multilingual sites require also the form field labels to be translated, however that does not work yet with sites < SS 2.3.2. (SS 2.3.2 creates a complete new form for each language so you recreate the fields and label them in the language).
There is a workaround to translate the labels of the fields which works for SS 2.3.1 sites and is especially useful for everyone using the [url=http://www.silverstripe.org/general-questions/show/253576?showPost=265161&start=0]Multilingual Content - alternative to 'translatable'[/url].
It requires activation of i18n and a small patch in /sapphire/forms/FormField.php.
All details are explained here [url=http://www.silverstripe.org/general-questions/show/253576?start=32#post265077]Workaround for the translation of form labels in SS 2.3.1[/url], however it is buried deeply in that thread there, so I post this note here to make it easier to find.
Hope it helps someone!