In general Thai police don’t hassle foreigners, especially tourists. They usually go out of their way to avoid having to speak English with a foreigner, especially regarding minor traffic issues. Thai police do, however, rigidly enforce laws against drug possession. Do be aware that some police divisions, especially on the Thai islands, might view foreigners and their legal infractions as a money-making opportunity.
If you are arrested for any offence, the police will allow you the opportunity to make a phone call, either to your embassy or consulate in Thailand if you have one, or to a friend or relative if not. There’s a whole set of legal codes governing the length of time and the manner in which you can be detained before being charged or put on trial, but a lot of discretion is left to the police. In the case of foreigners the police are more likely to bend these codes in your favour. However, as with police worldwide, if you don’t show respect you will make matters worse.
Thai law does not presume an indicted detainee to be either guilty or innocent but rather a ‘suspect’, whose guilt or innocence will be decided in court. Trials are usually speedy.
The tourist police can be very helpful in cases of arrest. Although they typically have no jurisdiction over the kinds of cases handled by regular cops, they may be able to help with translations or with contacting your embassy. You can call the hotline to lodge complaints or to request assistance with regards to personal safety.