If you are looking to modify the text or language settings for this specific GOG build:

Look for a language_setup.exe or similar utility within the game's installation folder. Running this allows you to toggle the interface and subtitle text between supported languages.

As a GOG release, this version does not require an internet connection or a launcher (like Steam) to run once installed.

Settings can often be manually adjusted in the config.lsx or settings.json files found in the Documents\Larian Studios\Divinity Original Sin 2 Definitive Edition directory.