The DITA OT does support an abstraction for loading language-specific versions of these types of "injected" phrases in a similar way to PDF. The files that contain the strings used for all the standard HTML-based outputs supported by the DITA OT are located in this folder:
<DITA OT>\xsl\common\If the root element of a particular topic (ie: <topic>, <task>, <reference>, etc) has this attribute set then the "Parent topic" text you refer to in your message will be inserted using the value from the corresponding file from the
<DITA OT>\xsl\common\ folder.
The particular file that is used for each xml:lang value is controlled by the settings in the file
<DITA_OT>\xsl\common\strings.xml, so to add new languages, or to map different xml:lang values to existing files you should modify that file.
If you are working with XMetaL Author Enterprise 5.5 or 6.0 and wish to make any changes to the DITA OT itself please see the following topic to be sure you are modifying the right copy of the DITA OT:
http://forums.xmetal.com/index.php/topic,237.0.html