Issues may arise when running reports containing Thai or other Unicode characters from an external database.
Thai characters in reports generated to the screen or any export format (such as PDF) may display as blank squares. This issue does not affect data stored in CRM in entity records, lists, etc.
The root cause of this issue can be the data source used for the Unicode characters. In the instance that was investigated, CRM was generating reports using a view that did an external JOIN onto an ERP database. The ERP database stored Thai script in a char column.
CASTing the data to an nvarchar in CRM's report view resolved the issue.
A good discussion on char and nchar columns is available here: