Changing DVD settings

[Marios14]

I'm looking for an app that will change settings of a ready made video_ts dvd folder (such as aspect ratio or adding subtitles) without recoding
the video and preserving the original menus.

[manono]

For the two things you mentioned the video doesn't have to be reencoded. For aspect ratio, PGCEdit can change that easily enough (as long as you mean 4:3->16:9 or vice-versa).

For adding subs you'd have to demux, remux with subs, and stick the 'fixed' DVD back into the original. Baldrick has a guide:

https://forum.videohelp.com/threads/277950-How-to-add-new-subtitles-to-an-existing-DVD

Use his Method 2.

[Marios14]

Can i also add a subtitle language pick entry on the original menu?

[manono]

Can i also add a subtitle language pick entry on the original menu?

Only with difficulty. I'd forget that one, if I were you, and either have the DVD play with your new subs automatically or just use the remote control subtitle button to turn them on and off.