Affirmative, of course it is possible to use SAP Business One in Thai Language.
Even SAP Business One is not available per default with Thai Language there are several convenient options for using it. One option is, as mentioned, to use a partner solution. Even simpler, you can create your own, even customer specific UI language terms. Please make yourself familiar with the Custom Language Tool as detailed in SAP note 1757764. It allows to define additional UI languages and make a new language available to SAP Business One users.
Same about the localization, even SAP Business One is not delivered as specifically localized for Thailand we have quite huge number of customers using it in Thailand, either in conjunction with partner add-on or based on another existing localization. For Thailand, the localization for Australia or Singapore is commonly used as basis. All this depends indeed on the specific use case and customer requirements. For more information, esp. on locally required legal reporting, please consult the local SAP contacts.
Hope this helps,
Please check the language options in your own B1 system. If Thai is in the list, then it is available.
Otherwise, I have heard of partners creating their own language file.
Also, if SAP doesn't offer a Thai localization, then the more important question is probably if B1 can be adjusted to adhere to Thai accounting legislation.