Thai Lion Air Seg, fev 04, 2019 Tóquio Narita, Japão NRT - 11: 00 Intl Don Mueang, Tailândia DMK - 04: 15 pm seg, fev 04, 2019 (tempo de espera 19hrs) ter, fev 05, 2019 Int. Mueang Don , Tailândia DMK - 11: 35 em Catmandu, Nepal KTM - 02: 00 pm ter, fev 05, 2019 (eu sou nepalês)
Você precisa de um visto enquanto está em trânsito por mais de uma hora do 12:
Passengers transiting at an airport in the Kingdom with a view to catching a scheduled connecting flight within 12 hours of their arrival without the intention of clearing immigration procedure, do not require a visa. They must however be inspected by the competent authorities upon arrival and must stay only in designated areas of the airport until their departure. All passengers transiting at an airport in the Kingdom for more than 12 hours must obtain a transit visa prior to their arrival.
(fonte)
Você pode ler detalhes da Embaixada Tailandia em Kathmandu, Nepal aqui.