Sure, no problem! But:

• Locale.setDefault is harsh! (for test it can be tolerable.... but i don't want to look up that bug..)
• (I "don't like" MessageFormat... it uses StringBuffer internally(Mr. performance killer & memory dumper).)

As long as you can handle the "complexity", it can be done like:

import java.math.BigDecimal;
import java.text.MessageFormat;
import java.text.NumberFormat;
import java.util.Locale;

public class MainSimple {

  // Brazilian Locale:
  private static final Locale LOCALE_PT_BR = new Locale.Builder().
      setLanguage("pt").
      setRegion("BR").
      build();
  // US number format:
  private static final NumberFormat DEFAULT_NUMBER_FMT = NumberFormat.getNumberInstance(Locale.US);

  public static void main(String[] args) {
    // MessageFormat instance, with format (with 2! parameters) and Locale:
    MessageFormat mainFmt = new MessageFormat("""
                      '{'
                          "value": {0},
                          "description": "O valor é {1,number,currency}. <-- trick 1 "
                      '}'
                      """, LOCALE_PT_BR);
    
    final BigDecimal num = new BigDecimal(946.2);
    // trick 2:
    Object[] params = {DEFAULT_NUMBER_FMT.format(num), num};
    // do it:
    System.out.println(mainFmt.format(params));
  }
}

Outline:

• Constants for:
  • Brazilian (outer format) locale.
  • US/Standard (inner) number format for custom (non brazilian) formatting/this particular number.
• MessageFormat has a constructor with locale parameter! (we use that, and call it outer format/locale(mainFmt))
• Int the format we "trick a little" and use 2 parameters: {0} and {1,number,currency}
• We trick on: Pass the first parameter as a (US-formatted number) String, the second as had. (Object[] params = {DEFAULT_NUMBER_FMT.format(num), num})
• finally format (& print to out).

Prints:

{
    "value": 946.2,
    "description": "O valor é R$ 946,20."
}

This is a String.format solution with two inner (localized, customized) NumberFormatters:

import java.math.BigDecimal;
import java.text.NumberFormat;
import java.util.Locale;

public class OtherMain {

  private static final Locale LOCALE_PT_BR = new Locale.Builder().
      setLanguage("pt").
      setRegion("BR").
      build();
  
  private static final NumberFormat DEFAULT_NUMBER_FMT
      = NumberFormat.getNumberInstance(Locale.US);
  private static final NumberFormat MY_SPECIAL_NUMBER_FMT
      = NumberFormat.getCurrencyInstance(LOCALE_PT_BR);

  static {
    // this disables "thousands sperarator", we can adjust DEFAULT_NUMBER_FMT as well...
    MY_SPECIAL_NUMBER_FMT.setGroupingUsed(false);
  }

  public static void main(String[] args) {
    final BigDecimal num = new BigDecimal(99946.2);
    System.out.format("""
                      {
                        "value": %s,
                        "description": "O valor é %s."
                      }
                      """,
        DEFAULT_NUMBER_FMT.format(num),
        MY_SPECIAL_NUMBER_FMT.format(num)
    );
  }
}

Prints:

{
  "value": 99,946.2,
  "description": "O valor é R$ 99946,20."
}