During the first boot or when the language field in options.txt is missing, Minecraft  defaults to English (US) instead of system language. For comparison Minecraft Launcher and Minecraft: Bedrock Editon will use system language if their settings are missing.
Steps to reproduce:

• Set your system language to be anything other than English (US).
• Delete the options.txt file in your Minecraft instance folder or set the lang parameter in it to nothing.
• Notice that Minecraft will use English (US) instead of your system language.