xaml export inconsistently translates unitless lengths ("e.g. "," as decimal separator) when in german locale

Migrated from: https://bugs.launchpad.net/inkscape/+bug/1789410 to inbox#2208 (closed) and then moved here

Steps to reproduce:

• Open Inkscape in a German locale (export LC_NUMERIC=de_DE.UTF-8)
• Open Bug1789410.svg
• Open Save As dialog
• Export as xaml

What happened?

• The StrokeThickness has a comma decimal separator when stroke-width is 6.28023789pt but decimal separator when stroke-width is just 6.28023789

What should have happened?

Consistently writes decimal separator (preferably . rather than in specific locale)

Inkscape version

• 1.1-dev (530a203f00, 2020-04-07) debug build Linux Mint 19.1
• 0.92.0 r15299 Windows 10

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Thomas W. (thomasw-mitutoyo-ctl) wrote that:

<xsl:decimal-format> element is required for format-number() function to work correctly.

MSDN decimal-format: https://msdn.microsoft.com/en-US/library/ms256229(v=vs.120).aspx

MSDN format-number(): https://msdn.microsoft.com/en-us/library/ms256225(v=vs.110).aspx

Below was an unsuccessful workaround (produced StrokeThickness="NaN")

    <xsl:attribute name="StrokeThickness">
      <xsl:value-of select="format-number($stroke_width, '#.#')" />
    </xsl:attribute>

A generic issue was fixed earlier by using format-number($cy - @ry, "#.#"), in https://bugs.launchpad.net/inkscape/+bug/380199 but the problem still exists in this case.