Unfortunately, we don't know which plugin you are talking about. So probably you need to change the key of the profile question, not the language value as you said.

To change a profile question key, first search for the existing key. To do so, go to the admin area > Users > Profile Questions. Perform a right mouse click on the question row you want to change and search for your debugger / development tools. For Firefox, have a look at this tutorial. In the code you'll find the attribute question_name="xxxxx" (replace xxxxx with the key of your question).

Now, visit your database (e.g. via phpMyAdmin) and make a backup. Then, search for the value xxxxx. Change all occurrences of this value to "match_sex", also if it's part of a string.