Mkvs typically use text based softsubs, so you should be able to extract the subtitle file and then just copy-paste to the google translator.

I'd download mkvtoolnix to do the job. Use mkvinfo to check which track is the subtitles, then mkvextract to get it out of the container. Do the translation, pray the format remains as it is (I wouldn't count on that), fix the formatting, mux back in, taking also out the old subtitles. You might want to use the mkvmerge gui for muxing, which is so simple it's idiot proof (I know because I can use it).

Mkvinfo and mkvextract are command line executables. So, you gotta bring back the good 'ol memories from times before guis (assuming you have those, being so young). They give you help with the switch -?. If you install the packet somewhere (like under program files (x86), you need to path it for easier access when you switch to the working folder containing the actual video file. (It's just path "C:\Program Files (x86)\MKVtoolnix" or wherever you installed it (the quotation marks are needed because there are spaces). This is so that the cmd finds the exes without needing to write the whole fricking path in front of them every time.

Still, I'm nothing but an amateur. Somebody wiser might have a more straightforward solution.