Summary:
Comparing in Meld doesn't work, because it cannot find g_getenv.

Additional info:
Package version mingw64/mingw-w64-x86_64-meld3 3.16.4-1

Steps to reproduce:

1. Launch meld from terminal.
2. Click "File comparison" in the middle.
3. Click "Blank comparison" below.

Expected result:
Meld should show two empty columns to insert text to compare.

Actual result:
Nothing happens in the GUI while it the terminal the errors below are printed.

(Note that some parts are in Polish, I tried changing the language to English by exporting LC_ALL and LANG environment variables set both to en_US.UTF-8 or C, without success.
Translation:
"Nie mo▒na odnale▒▒ okre▒lonego modu▒u" means "Cannot find given module"
"Nie można odnaleźć określonej procedury." means "Cannot find given procedure.")

The error log:

$ meld
Couldn't bind the translation domain. Some translations won't work.
[Error 126] Nie mo▒na odnale▒▒ okre▒lonego modu▒u
Traceback (most recent call last):
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/sourceview.py", line 155, in init
self.on_setting_changed(meldsettings, 'style-scheme')
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/sourceview.py", line 181, in on_setting_changed
"meld:current-line-highlight", "background")
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/misc.py", line 193, in colour_lookup_with_fallback
base_style = get_base_style_scheme()
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/misc.py", line 158, in get_base_style_scheme
env_theme = GLib.getenv('GTK_THEME')
GLib.Error: g-invoke-error-quark: Could not locate g_getenv: 'g_getenv': Nie można odnaleźć określonej procedury. (1)
Traceback (most recent call last):
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/sourceview.py", line 155, in init
self.on_setting_changed(meldsettings, 'style-scheme')
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/sourceview.py", line 181, in on_setting_changed
"meld:current-line-highlight", "background")
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/misc.py", line 193, in colour_lookup_with_fallback
base_style = get_base_style_scheme()
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/misc.py", line 158, in get_base_style_scheme
env_theme = GLib.getenv('GTK_THEME')
GLib.Error: g-invoke-error-quark: Could not locate g_getenv: 'g_getenv': Nie można odnaleźć określonej procedury. (1)
Traceback (most recent call last):
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/sourceview.py", line 155, in init
self.on_setting_changed(meldsettings, 'style-scheme')
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/sourceview.py", line 181, in on_setting_changed
"meld:current-line-highlight", "background")
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/misc.py", line 193, in colour_lookup_with_fallback
base_style = get_base_style_scheme()
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/misc.py", line 158, in get_base_style_scheme
env_theme = GLib.getenv('GTK_THEME')
GLib.Error: g-invoke-error-quark: Could not locate g_getenv: 'g_getenv': Nie można odnaleźć określonej procedury. (1)
Traceback (most recent call last):
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/diffmap.py", line 46, in init
self.on_setting_changed(meldsettings, 'style-scheme')
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/diffmap.py", line 78, in on_setting_changed
self.fill_colors, self.line_colors = get_common_theme()
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/misc.py", line 213, in get_common_theme
"insert": lookup("meld:insert", "background"),
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/misc.py", line 193, in colour_lookup_with_fallback
base_style = get_base_style_scheme()
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/misc.py", line 158, in get_base_style_scheme
env_theme = GLib.getenv('GTK_THEME')
GLib.Error: g-invoke-error-quark: Could not locate g_getenv: 'g_getenv': Nie można odnaleźć określonej procedury. (1)
Traceback (most recent call last):
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/diffmap.py", line 46, in init
self.on_setting_changed(meldsettings, 'style-scheme')
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/diffmap.py", line 78, in on_setting_changed
self.fill_colors, self.line_colors = get_common_theme()
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/misc.py", line 213, in get_common_theme
"insert": lookup("meld:insert", "background"),
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/misc.py", line 193, in colour_lookup_with_fallback
base_style = get_base_style_scheme()
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/misc.py", line 158, in get_base_style_scheme
env_theme = GLib.getenv('GTK_THEME')
GLib

Additionally when Meld closes, it prints these errors:

.Error: g-invoke-error-quark: Could not locate g_getenv: 'g_getenv': Nie można odnaleźć określonej procedury. (1)
Traceback (most recent call last):
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/newdifftab.py", line 128, in on_button_new_blank_clicked
tab = self.diff_methodsself.diff_type
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/meldwindow.py", line 691, in append_filediff
doc = filediff.FileDiff(len(files))
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/filediff.py", line 227, in init
self.set_num_panes(num_panes)
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/filediff.py", line 1789, in set_num_panes
w.associate(self, self.textview[i], self.textview[i + 1])
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/linkmap.py", line 45, in associate
self.on_setting_changed(meldsettings, 'style-scheme')
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/linkmap.py", line 49, in on_setting_changed
self.fill_colors, self.line_colors = get_common_theme()
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/misc.py", line 213, in get_common_theme
"insert": lookup("meld:insert", "background"),
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/misc.py", line 193, in colour_lookup_with_fallback
base_style = get_base_style_scheme()
File "D:/app/msys64/mingw64/lib/python2.7/site-packages/meld/misc.py", line 158, in get_base_style_scheme
env_theme = GLib.getenv('GTK_THEME')
GLib.Error: g-invoke-error-quark: Could not locate g_getenv: 'g_getenv': Nie można odnaleźć określonej procedury. (1)