The very first run needs to wait for the machine translation model to be downloaded so it won't be quick. For the subsequent runs, the model will be reused as a cache.
After installation, simply passing in the source and target language codes should do the job. E.g.,
$ subaligner -m dual -v video.mp4 -s subtitle.srt -t eng,spa
Or just translate without synchronisation:
$ subaligner_convert -i subtitle_en.srt -o subtitle_es.srt -t eng,spa