What I would do is somewhere in your test parameters, perhaps in a project level variable, set what language you're testing. So, say you're going to do a test run of English. You set your language variable to English.
What you would do then is set up your DDT file a little differently. I would add a column to your DDT file for language. Then, in your looping through the file, compare the record in the file to the global variable. if they match, proceed with the test, if they don't, skip it.
Then, when you want to test another language, change your global variable and try again.
This is only one way of doing it but you'll need to set up these parameters in some way to tell your test what is being tested. Will it take longer? You are adding more test cases so, yes, it will take longer however, the ability of TestComplete to compare the WndCaption to the value in the file will not take any longer since it's a simple string to string comparison.