Import both tracks into Audacity (File menu).
Split the Tamil track into sections so that you have a “split” each time a part is missing (http://manual.audacityteam.org/o/man/edit_menu_clip_boundaries.html#split)
Use the Time Shift tool to slide each section so that it is in the correct place with reference to the English track.
Export the finished project (File menu). Exporting will mix all tracks present in the Audacity project into one audio file.