The local video file feature currently only allows uploading a single SRT file. If it allowed uploading a secondary SRT file (containing translations), it could then reproduce the double subtitle support that the extension provides on youtube.com, including the feature to display the secondary subtitle only on an as-needed basis, after pressing the E key.
Something like this (but Language Reactor-ified to looking cleaner and work towards LR Learners) would be super helpful:
Source Credit: Images screen captured from: Font Settings And Subtitle Settings In Video Content - YouTube (all due credit to migaku.io).
Please and thank you!!!