Ok, finally it is not a bug, but missing translation texts for your language.
You need to translate this part of language pack:
; Supplements
QUANTITY="Quantity"
SUPPLEMENTS="Supplements"
LIST SUPPLEMENT FREE="%s"
YESNO SUPPLEMENT FREE="for free"
YESNO SUPPLEMENT BOTH MULTIPLY="%s (%s * %s * capacity)"
LIST SUPPLEMENT BOTH MULTIPLY="%s: %s (%s * %s * capacity)"
YESNO SUPPLEMENT CAPA MULTIPLY="%s (%s * capacity)"
LIST SUPPLEMENT CAPA MULTIPLY="%s: %s (%s * capacity)"
YESNO SUPPLEMENT UNIT MULTIPLY="%s (%s * %s)"
LIST SUPPLEMENT UNIT MULTIPLY="%s: %s (%s * %s)"
YESNO SUPPLEMENT FREE MULTIPLY="%s"
LIST SUPPLEMENT FREE MULTIPLY="%s: %s"
SUPPLEMENT LABEL PRICE="%s (%s)"
SUPPLEMENT LABEL PRICE CAPACITY="%3$sx %s: %s (%s x %s)"
SUPPLEMENT FREE LABEL PRICE="%s (free)"
YESNO SUPPLEMENT LABEL PRICE="Yes (%s)"
YESNO SUPPLEMENT LABEL PRICE CAPACITY="%2$sx: %s (%s x %s)"
SUPPLEMENT FREE PRICE="Yes (free)"
RESERVATION PRICE="Reservation price"
RESERVATION DEPOSIT="Reservation deposit"
MINIMUM CAPACITY="Minimum capacity"
MINIMUM CAPACITY INFO="Minimum capacity to start event. If minimal capacity in certain interval is not fullfilled, event not will be performed. This is only informative."