AllTalk TTS: Czech diacritics lost (non-UTF8 form encoding in request)

Start at the AllTalk TTS integration and trace the POST request sent to /api/tts-generate, comparing its form-body encoding and Content-Type with the working PowerShell request. Reproduce with the Czech sample text and verify that AllTalk receives and preserves ř, č, ž, and other diacritics.

Subtitle Edit version: [Subtitle Edit 4.0.14]
Windows: [doplň verzi - např. Windows 10 22H2]
AllTalk: [v1.9c] running on http://127.0.0.1:7851
TTS Engine: AllTalk TTS (Coqui XTTSv2 based)
Language: Czech (cs)

Problem

When generating TTS via AllTalk from Subtitle Edit, Czech diacritics (ř, č, ž, etc.) are corrupted.

Example input in Subtitle Edit:

Takže čeština tu funguje suprově, co ty tomu říkáš?

What AllTalk receives (visible in console log):

Take etina tu funguje suprov, co ty tomu íká?

However, AllTalk itself handles Czech correctly when called directly via API.

Evidence / Reproduction

Calling AllTalk API directly via PowerShell works correctly and preserves diacritics:

$body = @{
  text_input           = "Takže čeština tu funguje suprově, co ty tomu říkáš?"
  text_filtering       = "none"
  character_voice_gen  = "female_01.wav"
  narrator_enabled     = "false"
  narrator_voice_gen   = "male_01.wav"
  text_not_inside      = "character"
  language             = "cs"
  output_file_name     = "ps_test"
  output_file_timestamp= "true"
  autoplay             = "false"
  autoplay_volume      = "0.8"
}

Invoke-RestMethod -Method Post `
  -Uri "http://127.0.0.1:7851/api/tts-generate" `
  -ContentType "application/x-www-form-urlencoded; charset=utf-8" `
  -Body $body
...

AllTalk console output (correct):

[AllTalk TTSGen] Takže čeština tu funguje suprově, co ty tomu říkáš?

Root Cause (suspected)

Subtitle Edit likely encodes the POST request body using the system codepage
(Encoding.Default / CP-1250) instead of UTF-8 when sending
application/x-www-form-urlencoded data, causing non-ASCII characters
to be corrupted before reaching AllTalk.

Suggested Fix

Ensure UTF-8 encoding is used for application/x-www-form-urlencoded POST bodies
and explicitly set Content-Type: application/x-www-form-urlencoded; charset=utf-8
in the HTTP request headers.

Related

Similar encoding issues have been reported in:

• #3384
• #8686