https://invent.kde.org/multimedia/subtitlecomposer

An open source text-based subtitle editor that supports basic and advanced editing operations.
Text subtitle formats: SubRip (SRT), MicroDVD, SSA/ASS, MPlayer, TMPlayer and YouTube captions
Graphics subtitle formats: VobSub (.idx/.sub/.rar), BluRay/PGS (*.sup), DVD/BluRay (DVD/Vob, DVB, XSUB, HDMV-PGS)
Video player and Waveform preview/editor
Speech Recognition; Translations; Easy subtitle sync; Spell checking;
Scripting (JavaScript, Python, Ruby)