In this case, I think you could just use the MODX - lexicon to translate the labels
or create two form-chunks, one for each language with names like
form_de and form_en
and call them with
[[!$myform_[[++cultureKey]]]]
you really don't need to create template-variables for them.
A third solution could be, to create your form with MIGX
Have two fields in the MIGX like
label_de and label_en
and let MIGX (getImageList) generate your form and have the translated labels with
[[!+label_[[++cultureKey]]]]