Non unicode single quote problem and solution

While doing XSLT transformation, I have experienced a strange problem. The single quote ' stored in the data dictionary has been transformed to non-unicode single quote e.g. ‘ and ’. I still don't know the reason for it as the output xml is in UTF-8. But here is the solution:

<xsl:variable name="singlequote" select='"&apos;"'/>

<xsl:value-of select="translate(translate($ddDoc/IRIS_Data_Dict/instrument/typeOfInstruments//type/@def[../@value=$currentType],'‘',$singlequote),'’',$singlequote)"/>