If your data contains text with non-latin characters, e.g., Asian language characters, you must make sure that the font you use for the grids in DbVisualizer contains those characters.
Open Tools->Tool Properties and select General / Appearance / Fonts. Change the selection for Grid to an appropriate font. You may need to search the net for the name of a font that contains the characters you need. For instance, on Windows the SimSun font should work for Chinese characters.
Even if the font is capable of rendering the characters, data may scrambled in DbVisualizer. The most common reason is that the encoding between the JDBC driver and the database is wrong. Please check the JDBC driver documentation and if there is a driver property available to set an alternative encoding.