extensions: fix localization of the Python 3 goat exercise.

We must set the correct gettext domain and bind it to the appropriate
directory for it to be localized appropriately.
16 jobs for master in 34 minutes and 21 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Prepare
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build-image

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passed #945000
build-image-win32

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passed #944999
build-image-win64

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  Dependencies
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deps-debian

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deps-win32

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passed #945002
deps-win64

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  Gimp
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gimp-autotools-debian

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passed #945007
gimp-clang-debian

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gimp-distcheck-debian

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passed #945006
gimp-meson-debian

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passed #945009
gimp-win32

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passed #945008
gimp-win64

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  Analysis
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cppcheck

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  Distribution
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sources

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passed #945013
win32-nightly

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passed #945012
win64-nightly

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