Assuming that the plan handles this properly, the outstanding loan balance will be reported as an offset distribution, reducing the total value of your investment in the plan by the amount of the offset. The plan rules will dictate how much of the distribution is attributable to the balance in the traditional 401(k) account and how much is attributable to a distribution from the Roth account. Plan rules often require that distributions be made from each account in proportion to each accounts fraction of the overall plan balance.
As indicated in the link that SweetieJean provided, distributions from the Roth 401(k) account will be a mix of contributions and earnings in proportion to each one's fraction of the balance in the Roth IRA. This means that if you have not had the Roth 401(k) for 5 years from the beginning of the year in which you first made a Roth 401(k) deferral or In-plan Roth Rollover or you have not reached age 59½, the earnings will be taxable and, if your are under age 59½, subject to an early-distribution penalty. Similarly, the distribution from the traditional 401(k) account will be subject to tax and possible early-distribution penalty. You also have state taxes and possible penalties to consider.
If the plan requires the distribution to be a mix of traditional and Roth account distributions, since an offset distribution is eligible for rollover, you can shift the mix by performing a rollovers. However, the plan is required to withhold a minimum of 20% for taxes on the taxable part of any eligible rollover distribution, and possibly withhold state taxes, increasing the amount of a distribution, so it might be difficult to complete the necessary rollovers without a source of additional funds (a source which seems unlikely to be available if you have reason not to pay off the loan). The net result is that not repaying the loan could be extremely expensive in terms of penalties and lost earning potential.
