lvm --reportformat json_std generates invalid JSON for percentages in locales with radix character = ","

Using --reportformat json_std leads to invalid JSON when the locale specifies "," (comma) as the radix character (e.g. de_DE, nl_NL, etc.):

# LANG=en_GB.UTF-8 lvs --report-format json_std -o data_percent fedora/home 
  {
      "report": [
          {
              "lv": [
                  {"data_percent":11.49}
              ]
          }
      ]
      ,
      "log": [
      ]
  }

Vs.:

# LANG=de_DE.UTF-8 lvs --report-format json_std -o data_percent fedora/home 
  {
      "report": [
          {
              "lv": [
                  {"data_percent":11,49}
              ]
          }
      ]
      ,
      "log": [
      ]
  }

The latter is not syntactically valid JSON, since , (0x2C) is the value-separator, and numbers require the decimal-point to be "." (0x2E):

https://www.rfc-editor.org/rfc/rfc7158#section-6

Common parsers are unable to parse the resulting strings:

# LANG=de_DE.UTF-8 lvs --report-format json_std -o data_percent fedora/home | python3 -c "import sys; from json import loads; loads(sys.stdin.read())"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
    import sys; from json import loads; loads(sys.stdin.read())
                                        ~~~~~^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.13/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
           ~~~~~~~~~~~~~~~~~~~~~~~^^^
  File "/usr/lib64/python3.13/json/decoder.py", line 345, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
               ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.13/json/decoder.py", line 361, in raw_decode
    obj, end = self.scan_once(s, idx)
               ~~~~~~~~~~~~~~^^^^^^^^
json.decoder.JSONDecodeError: Expecting property name enclosed in double quotes: line 5 column 38 (char 93)

This isn't an issue for --reportformat json, since the offending value is encoded as a quoted string in _output_field_json_fmt(). With json_std the percent field is treated as a pure numeric field and encoded as a JSON number using dm_report_field_percent().