webtrees is based on the GEDCOM 5.5.1 specs, which does not explicitly state what types of marriage are allowed. The type of a marriage is considered free text according to the specs. There is a new 5.5.5 spec that has a list of suggested marriage types:
. The nature of the relationship is documented by the optional MARR.TYPE subrecord, and GEDCOM 5.5.5 provides a list of the common MARR.TYPE values and their meaning in the definition of the <<FAM_GROUP_RECORD>>.
unknown relationship (type unknown)
not married not married
civil civil marriage
religious religious marriage
common law common law marriage
registered partnership registered partnership
living together living together
living apart together living apart together
I can understand why "Samenlevingscontract" is the preferred & chosen option in webtrees, since "Geregistreerd Partnerschap" quite often is a same-sex marriage. Yes, it is no rule. I am not married but registered partner without a cohabitation contract with my wife myself!
I have the impression that the current list of marriage types is not straight forward to use for English users that might expect common law, civil partnership or whatever they call it. Other countries and cultures will have similar struggles.
One problem with changing an existing translation is that there are dozens of users that have picked the Dutch type 'Samenlevingscontract' from the drop-down list for ages, and upon an update will be confronted with an unwanted change of terminology.