It is possible to lengthen the silence between words or phrases with a plug-in called “silencelength.ny”. This works on a Mac, but I don’t know if it will work on a PC.
I found this plug-in somewhere on the internet several years ago and used it successfully with Audacity 1.3.
Later when I upgraded to Audacity 2.02 the option to lengthen silences had gone, but I managed to solve it by copying the file called “silencelength.ny” from my old folder called audacity ver 1_3/plug-ins to the folder called Audacity ver 2_0_2/plug-ins. The audacity program folder should be in your applications folder.
I think you can find the plug-in here, but I can’t guarantee its the same one I found:
https://forum.audacityteam.org/t/lengthen-silences/12326/1
There seems to be a bug in my version. It wont work with Stereo tracks so first you need to convert the track to mono by clicking Tracks - Stero Track to Mono.
To launch the plug in, re-start audacity (from your applications folder) and then open any MONO track (or convert a stereo one to mono) and then click ==> Effect==>Silence Lengthen. You then choose what audio level constitutes “silence” (the default of 26db works fine), and the minimum duration which constitutes a silence (default is 0.3s of silence), and then set the number of secondes to insert. For learning languages, I insert 2.5 seconds.