gcolor3 is a GTK base color picker.
Unfortunately, the upstream source 2.3.1 does not support Japanese language.

Here is my personal package of gcolor3-2.3.1 with Japanese language support for slackware64-15.0.

1. Source
gcolor3.info
[code]
PRGNAM="gcolor3"
VERSION="2.3.1"
HOMEPAGE="https://gitlab.gnome.org/World/gcolor3"
DOWNLOAD="https://gitlab.gnome.org/World/gcolor3/-/archive/v2.3.1/gcolor3-v2.3.1.tar.gz"
MD5SUM="a611dbb3ee2561dbc70eb59aa473b10d"
DOWNLOAD_x86_64="direct download link(s) of application source tarball(s), x86_64 only"
MD5SUM_x86_64=""
REQUIRES="%README%"
MAINTAINER="JW(Shinichi Abe)"
EMAIL="shin1.abe@nifty.com"

2. SlackBuild
gcolor3.SlackBuild
[code]
#!/bin/sh

# Slackware build script for gcolor3

# Copyright 2021 JW(Shinichi Abe) <shin1.abe@nifty.com>
# All rights reserved.
#
# Redistribution and use of this script, with or without modification, is
# permitted provided that the following conditions are met:
#
# 1. Redistributions of this script must retain the above copyright
#    notice, this list of conditions and the following disclaimer.
#
#  THIS SOFTWARE IS PROVIDED BY THE AUTHOR "AS IS" AND ANY EXPRESS OR IMPLIED
#  WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
#  MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO
#  EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
#  SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
#  PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
#  OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
#  WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
#  OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
#  ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

PRGNAM=gcolor3
VERSION=${VERSION:-2.3.1}
BUILD=${BUILD:-1}
TAG=${TAG:-jw}

if [ -z "$ARCH" ]; then
  case "$( uname -m )" in
    i?86) ARCH=i586 ;;
    arm*) ARCH=arm ;;
       *) ARCH=$( uname -m ) ;;
  esac
fi

CWD=$(pwd)
TMP=${TMP:-/tmp/SBo}
PKG=$TMP/package-$PRGNAM
OUTPUT=${OUTPUT:-/tmp}

if [ "$ARCH" = "i586" ]; then
  SLKCFLAGS="-O2 -march=i586 -mtune=i686"
  LIBDIRSUFFIX=""
elif [ "$ARCH" = "i686" ]; then
  SLKCFLAGS="-O2 -march=i686 -mtune=i686"
  LIBDIRSUFFIX=""
elif [ "$ARCH" = "x86_64" ]; then
  SLKCFLAGS="-O2 -fPIC"
  LIBDIRSUFFIX="64"
else
  SLKCFLAGS="-O2"
  LIBDIRSUFFIX=""
fi

set -e

rm -rf $PKG
mkdir -p $TMP $PKG $OUTPUT
cd $TMP
rm -rf $PRGNAM-$VERSION
tar xvf $CWD/$PRGNAM-$VERSION.tar.?z
cd $PRGNAM-$VERSION
# Apply debian patch (fix eye-drop picker)
zcat $CWD/G_PARAM_PRIVATE-deprecation.patch.gz | patch -p1 --verbose || exit 1
# Apply Japanese translation
zcat $CWD/gcolor3-Add_ja.po_v1.2.patch.gz | patch -p1 --verbose || exit 1


chown -R root:root .
find -L . \
 \( -perm 777 -o -perm 775 -o -perm 750 -o -perm 711 -o -perm 555 \
  -o -perm 511 \) -exec chmod 755 {} \; -o \
 \( -perm 666 -o -perm 664 -o -perm 640 -o -perm 600 -o -perm 444 \
  -o -perm 440 -o -perm 400 \) -exec chmod 644 {} \;

mkdir build
cd build
  CFLAGS="$SLKCFLAGS" \
  CXXFLAGS="$SLKCFLAGS" \
  meson .. \
    --buildtype=release \
    --infodir=/usr/info \
    --libdir=/usr/lib${LIBDIRSUFFIX} \
    --localstatedir=/var \
    --mandir=/usr/man \
    --prefix=/usr \
    --sysconfdir=/etc
  ninja
  DESTDIR=$PKG ninja install
cd ..

find $PKG -print0 | xargs -0 file | grep -e "executable" -e "shared object" | grep ELF \
  | cut -f 1 -d : | xargs strip --strip-unneeded 2> /dev/null || true

find $PKG/usr/man -type f -exec gzip -9 {} \;
for i in $( find $PKG/usr/man -type l ) ; do ln -s $( readlink $i ).gz $i.gz ; rm $i ; done

#rm -f $PKG/usr/info/dir
#gzip -9 $PKG/usr/info/*.info*

find $PKG -name perllocal.pod \
  -o -name ".packlist" \
  -o -name "*.bs" \
  | xargs rm -f

mkdir -p $PKG/usr/share/pixmaps
cp -a $CWD/nl.hjdskes.gcolor3.png $PKG/usr/share/pixmaps/nl.hjdskes.gcolor3.png

mkdir -p $PKG/usr/doc/$PRGNAM-$VERSION
cp -a \
  LICENSE  $PKG/usr/doc/$PRGNAM-$VERSION
cat $CWD/$PRGNAM.SlackBuild > $PKG/usr/doc/$PRGNAM-$VERSION/$PRGNAM.SlackBuild

mkdir -p $PKG/install
cat $CWD/slack-desc > $PKG/install/slack-desc
cat $CWD/doinst.sh > $PKG/install/doinst.sh

cd $PKG
/sbin/makepkg -l y -c n $OUTPUT/$PRGNAM-$VERSION-$ARCH-$BUILD$TAG.${PKGTYPE:-txz}


3. nosrc file which includes build-script and patches

This file includs ..
  - README
  - slack-desc
  - doinst.sh
  - gcolor3.info
  - gcolor3.SlackBuild
  - G_PARAM_PRIVATE-deprecation.patch.gz 
  - gcolor3-Add_ja.po_v1.2.patch.gz 
  - nl.hjdskes.gcolor3.png


4. Screenshot
gcolor3_w_Japanese_LC_Messages_2022-05-06