This script recursively sets the default language for all MKVs in the path it is executed in. It also unsets the default flag for any subtitles.

default_language.py

import pathlib
import subprocess
import simplejson as json

files = [*pathlib.Path('.').glob('**/*.mkv')]
total_items = len(files)
unprocessed_items = []
languages = {}

for file_index, file in enumerate(files):
    mediainfo = subprocess.run(['mediainfo', file, '--Output=JSON'], capture_output=True, text=True).stdout
    parsed_mediainfo = json.loads(mediainfo)
    tracks = parsed_mediainfo['media']['track']
    audio_tracks = [i for i in tracks if i['@type']  == 'Audio']
    subtitle_tracks = [i for i in tracks if i['@type'] == 'Text']

    # Calculating the percentage based on the current index, casting
    # the resulting float to a string and cutting it to 2 decimals.
    progress_percentage = f'{(file_index + 1) / total_items * 100:.2f}'
    progress = f'Processed {progress_percentage}% of {total_items} items'

    if len(audio_tracks) < 2:
        unprocessed_items.append(file)
        print(progress)
        continue

    args = []

    for audio_track in audio_tracks:
        language = audio_track['Language']
        uid = audio_track['UniqueID']
        is_default_track = audio_track['Default'] == 'Yes'
        # Taking advantage of the uniqueness of keys in a dictionary to map all the found languages.
        languages[language] = ''
        if language == 'en' and not is_default_track:
            args.append(f'--edit track:={uid} --set flag-default=1')
        elif language != 'en' and is_default_track:
            args.append(f'--edit track:={uid} --set flag-default=0')

    for subtitle_track in subtitle_tracks:
        is_default_track = subtitle_track['Default'] == 'Yes'
        if not is_default_track:
            continue
        uid = subtitle_track['UniqueID']
        args.append(f'--edit track:={uid} --set flag-default=0')

    if len(args) == 0:
        print(progress)
        continue

    command = ['mkvpropedit', f'"{file}"']
    command.extend(args)

    separator = " "

    command = separator.join(command)

    subprocess.run(command, shell=True)

    print(progress)

print('\nFound languages:')
for language in languages:
    print(f'- {language}')

if len(unprocessed_items) > 0:
    print('\nFound items with no second language:')
    for unprocessed_item in unprocessed_items:
        print(f'- {str(unprocessed_item)}')

requirements.txt

simplejson==3.20.1