Hmm sorry not going to answer all of your questions in detail.
When you create your display texts you can set the text (subtitles) for the various available languages by clicking on the flag (country) icon & selecting the language you want to write the text for.
Alternatively you can export the texts & back in at a later date using the export & import text options. I don't know about po editor thing. I believe I exported them in csv format & opened them up in open office or something - I forget which program I opened them up in off the top of my head.
If you are only providing one speech track then you only need to add the speech file to the primary language audio bit in each display text as it's possible via Lua script (since 4.x) to change the speech & text languages independently from one another.
game.StandardLanguage = Languages["English"] -- define text language
game.SpeechLanguage = Languages["Czech"] -- define speech language