How to restrict flutter Textfield or TextFormField to accept only english language?

Try with this

TextField(
  inputFormatters: [
            FilteringTextInputFormatter.allow(RegExp("^[\u0000-\u007F]+\$"))
    ])

Or you can try with this if you want only english character.

 TextField(
   inputFormatters: [
            FilteringTextInputFormatter.allow(RegExp("[a-zA-Z]"))
       ])

TextField(
    inputFormatters: <TextInputFormatter>[
       FilteringTextInputFormatter.allow(RegExp('[a-z A-Z 0-9]'))
    ],
)

You can use regular expressions of english

bool validEnglish(String value) { 
  RegExp regex = RegExp(r'/^[A-Za-z0-9]*$'); 
  return (!regex.hasMatch(value)) ? false : true; 
}

TextFormField(
              decoration: InputDecoration(
                hintText: 'Enter text',
              ),
              textAlign: TextAlign.center,
               validator: (text) {
                if (text == null || text.isEmpty || !validEnglish(text)) {
                  return 'Text is empty or invalid' ;
                }
                return null;
              },
            )