This is my html form:
    <div class="custom-form">
      <h1>문의 양식</h1>
      <p>아래 양식을 기입하신 후, 제출을 클릭해 주시면 담당자가 확인하고 연락을 드립니다.</p>
      
      <form action="http://127.0.0.1:5000/submit_form" method="POST">
        <div class="form-group">
          <label for="name">이름:</label>
          <input type="text" id="name" name="name" required>
        </div>
        <div class="form-group">
          <label>성별:</label>
          <div class="form-group inline">
            <input type="radio" id="male" name="gender" value="남성" required>
            <label for="male">남성</label>
            <input type="radio" id="female" name="gender" value="여성" required>
            <label for="female">여성</label>
          </div>
        </div>
        <div class="form-group">
          <label for="age">나이:</label>
          <input type="number" id="age" name="age" min="1" required>
        </div>
        <div class="form-group">
          <label for="admission-date">입학을 원하는 시기:</label>
          <input type="text" id="admission-date" name="admission_date" placeholder="예: 2024년 3월" required>
        </div>
        <div class="form-group">
          <label for="current-school">현재 다니는 학교:</label>
          <input type="text" id="current-school" name="current_school" required>
        </div>
        <div class="form-group">
          <label for="english-level">영어 이해도:</label>
          <select id="english-level" name="english_level">
            <option value="상">상</option>
            <option value="중">중</option>
            <option value="하">하</option>
          </select>
        </div>
        <div class="form-group">
          <label for="questions">기타 질문:</label>
          <textarea id="questions" name="questions" rows="4" placeholder="여기에 질문을 입력하세요."></textarea>
        </div>
        <div class="form-group">
          <label for="contact-preference">연락 받기를 원하는 방법을 기입 (이메일 또는 전화):</label>
          <input type="text" id="contact-preference" name="contact_preference" placeholder="예: scis@outlook.kr 혹은 031-548-2198" required>
        </div>
        <button type="submit">제출</button>
      </form>
      <footer>
        <p>문의 사항이 있으시면 scis@outlook.kr로 연락주세요.</p>
      </footer>
    </div>

And this is backend:
from flask import Flask, request, jsonify
import smtplib
from email.mime.text import MIMEText
from email.mime.multipart import MIMEMultipart

app = Flask(__name__)

# Function to send an email
def send_email(to_email, subject, body):
    smtp_server = "smtp.gmail.com"  # SMTP server (Gmail in this case)
    smtp_port = 587  # SMTP port
    sender_email = "secret"  # Replace with your email
    sender_password = "secret"  # Replace with your email password or app-specific password

    try:
        # Create the email
        msg = MIMEMultipart()
        msg['From'] = sender_email
        msg['To'] = to_email
        msg['Subject'] = subject
        msg.attach(MIMEText(body, 'plain'))

        # Connect to the SMTP server and send the email
        server = smtplib.SMTP(smtp_server, smtp_port)
        server.starttls()  # Upgrade the connection to secure (TLS)
        server.login(sender_email, sender_password)
        server.send_message(msg)
        server.quit()
        return True
    except Exception as e:
        print(f"Error: {e}")
        return False

@app.route('/submit_form', methods=['POST'])
def submit_form():
    try:
        # Get form data from the request
        form_data = request.form
        name = form_data.get('이름')  # Use Korean key '이름'
        gender = form_data.get('성별')  # Use Korean key '성별'
        age = form_data.get('나이')  # Use Korean key '나이'
        admission_date = form_data.get('입학을 원하는 시기')  # Use Korean key
        current_school = form_data.get('현재 다니는 학교')  # Use Korean key
        english_level = form_data.get('영어 이해도')  # Use Korean key
        questions = form_data.get('기타 질문')  # Use Korean key
        contact_preference = form_data.get('연락 방법')  # Use Korean key

        # Prepare email content
        subject = "새로운 양식 제출"
        body = f"""
        새로운 양식 제출이 접수되었습니다:

        이름: {name}
        성별: {gender}
        나이: {age}
        입학을 원하는 시기: {admission_date}
        현재 다니는 학교: {current_school}
        영어 이해도: {english_level}
        기타 질문: {questions}
        연락 방법: {contact_preference}
        """

        # Send email
        recipient_email = "mangostin2010@gmail.com"  # Replace with your desired recipient email
        if send_email(recipient_email, subject, body):
            return jsonify({"message": "Form submitted and email sent successfully!"}), 200
        else:
            return jsonify({"message": "Failed to send email."}), 500
    except Exception as e:
        return jsonify({"error": str(e)}), 500

# Run the Flask application
if __name__ == '__main__':
    app.run(debug=True)

But the email sent is like
이름: None
성별: None
...