If it is impossible to reach a good compromise, and they are forced to come in justice, they should go before a Jewish court [of halakhah]. It is prohibited to stand judgment before non-Jewish judges and their adjudication even in laws that they just the same as Jewish law. Even if both litigants want to stand judgment before them, it is prohibited. Even if the purchase was tied to this condition or they wrote it in a contract [that they would go to non-Jews for any adjudication], it is nothing. Whomever comes to judgement fefore them, he is an evil person, and it’s as though he got angry and rebelled and lifted a hand against the Torah of Moshe Rabbeinu a”h. Even something which he is permitted to serve justice on his own, as I will explain — if G-d desires — in se’if 9, in any case one may not do so through the aegis of non-Jews. Even if he is not judged by non-Jews but he was compelled by non-Jews to stands with him for Jewish justice, it would be appropriate to stretch him against the pillar [for lashes].
The notion of law is central to Judaism. It is therefore tantamount to denying Judaism for someone to imply that Jewish law is insufficient and they need to go elsewhere.
In practice, though, the above is only true where Jewish law is actually sufficient. In cases where our current exile makes it impossible for a beis din to enforce the law (e.g. charging someone with child molestation ), or the people are noted in the community and it would be difficult to find a court that both parties agree is unbiased, it may be permissible. Consult your rabbi if this ch”v were to arise.