I don't think your workstation is the problem so much as Kdenlive is the problem.
I get the feeling also... But the choice of editing software on Linux is quite limited... And since Adobe changed to that renting model which basically requires the editing machine to have an internet connection, that's out of the question for me (besides the fact that it's out of my budget).
Subtitles can be created in any text editor. (Unless you want DVD subtitles, you have to convert them to that DVD image subtitle format).
I think, creating them in a text editor is fairly unintuitive. Aegisub (what I mentioned before) allows basically "on screen" WYSIWYG editing.
If he's only doing translations, then you'd want to pre-subtitle it in whatever language with timing, and then he can just translate that text.
Isn't that kinda double work? She does understand/speak German and English perfectly fluent, so my idea was basically, to tell her what software to set up on her PC and then send her a USB-SSD with the material to work on. She would then send the SSA/ASS/SRT/whatever file back to me.
My main issue is: In case a TV station or production service wants the material - which format are they going to want the subs in and how to I create that format? DO they nowadays still use the EBU STL format?
In Mac? Plenty of them, Open Source and free!
Actually, Linux or Windows but if it's Open Source, chances are it's possible to get it to run and/or compile on Linux. Do you have recommendations for good software which creates files which could be accepted by "professionals"?