FIX: make EngFormatter respect axes.formatter.use_locale rcParam - #32281

Description

EngFormatter ignores axes.formatter.use_locale when formatting the mantissa.

With de_DE.UTF-8:

import locale

import matplotlib as mpl
from matplotlib.ticker import EngFormatter, ScalarFormatter

locale.setlocale(locale.LC_ALL, "de_DE.UTF-8")

with mpl.rc_context({"axes.formatter.use_locale": True}):
    scalar = ScalarFormatter()
    eng = EngFormatter(places=2)

ScalarFormatter.get_useLocale(): True
EngFormatter.get_useLocale()   : False

ScalarFormatter.format_data(0.5551): '5,551e−1'
EngFormatter.format_data(0.5551)   : '555.10 m'

There are two causes: EngFormatter.__init__ passes useLocale=False to ScalarFormatter, and format_data formats the mantissa directly instead of using locale-aware formatting. Because of the latter, even set_useLocale(True) does not affect the normal format_data path. Locale-aware formatting is currently only used for the offset path.

This change passes useLocale through to ScalarFormatter, adds it as an EngFormatter keyword argument, and applies locale formatting to the mantissa. For MathText, locale separators are escaped to avoid punctuation spacing.

Tests cover the rcParam, explicit useLocale values, the inherited setter, and MathText.

I consider this a bug fix: EngFormatter already inherits the locale API from ScalarFormatter, but does not honor it consistently. #13477 fixed the same kind of issue for axes.unicode_minus.

The new constructor keyword is the only public API addition here and can be split out if preferred.

Related: #25006 concerns the same rcParam, but a different issue involving locale initialization/reset behavior.

PR quality check

• Use an expressive title, e.g. "Fix title font property precedence"
• New and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
• New features and API changes have release notes
• Documentation complies with general and docstring guidelines