There are extremely few churches in Thailand that hold to explicitly Reformed theology, although there are a number of missionaries and Thai Christians who are Calvinistic or Gospel-centered/Christ-centered to some degree, scattered throughout various churches and mission organizations in Thailand.
In Bangkok, there are no Reformed churches with international services that are completely in English. If you are looking for a completely English language service, please see my list here.
If you'd like to visit a Thai church, there are two Reformed churches in Bangkok and one in Chiang Mai that I know and can recommend. Both have connections to Mission to the World (MTW), the denominational mission board of the Presbyterian Church in America.
This church is in southeastern Bangkok, right next to Ramkhamheng University 2 campus. They worship on Sunday mornings in Thai, and there is sometimes translation of the sermon for non-Thai speaking visitors.
This is a new church plant since 2013, and is currently meeting for worship at 4pm on Sunday afternoons. This is a Thai-speaking church, although there are a large amount of bi-lingual folks there, both Thai and foreign, and the sermon translation into English is available.
GracePoint is Reformed church plant in Chiang Mai, the biggest city in Northern Thailand. The church uses Thai as the primary language in worship though there are some English speakers among the leaders and congregation.