The language used by Premiere Pro is decided by the user during installation.

Plug-ins can determine this setting from the following locations:

On Windows, in the registry at HKEY_CURRENT_USER\Software\Adobe\Premiere Pro\[version], in a key named "Language".

On macOS, at ~/Library/Preferences/com.Adobe.Premiere Pro.[version].plist, at Root > Language.

The string will be set to one of the values below by Premiere Pro at startup.

Language String
English en_US
French fr_FR
German de_DE
Italian it_IT
Japanese ja_JP
Spanish es_ES
Korean ko_KR
Chinese (new in CC) zh_CN
Russian (new in CC 2014) ru_RU
Brazilian Portugese (new in CC 2014) pt_BR

Changing the string will not change the language Premiere Pro runs in, unless you override the application language by placing a file in the following location:

Windows: [App installation folder]\lang-override.txt

macOS: [App Installation folder]/[Premiere Pro application package]/Contents/lang-override.txt