This could of went into Slackware forum but this seems to off happened with other distro's I have used as well & I want to find out what has been causing this.
I did an upgrade of different software using GSlapt & afterwards found that now most software that I open thats not KDE related but seems to be GTK related will be unreadable with rectangles.
The KDE apps seem to be fine, but apps like Gslapt,wicd,lxappearance etc show rectangles in stead of words & numbers.
I checked "/etc/profile.d/lang.sh" & it seemed be the same as before.

If you are using Slackware, did you do a complete install?

I installed from a Live disc which I think was Slackel & have had it installed for maybe a couple of months or more

Obviously GTK doesn't have a valid font name, although I can't guess why. You can set one with a command like
gtk-font-name="sans-serif 8"
but where to put it? Try ~/.gtkrc-2.0 but no guarantees are offered!

This *might* put you on the road to a solution:

Good Luck.

I Have already Oxygen-Gtk2 & Oxygen-Gtk3 installed and .gtkrc-2.0 was already setup that way.
I am at a loss why this keeps happening

I think I have found the Culprit.
I tried starting some of these apps from konsole & I am getting a "Pango" error like this

1)
me2[/]$ lxappearance
/home/me2/.gtkrc-2.0:6: error: unexpected identifier `Sans', expected character `='

(lxappearance:10222): Pango-CRITICAL **: No modules found:
No builtin or dynamically loaded modules were found.
PangoFc will not work correctly.
This probably means there was an error in the creation of:
'/etc/pango/i486-slackware-linux/pango.modules'
You should create this file by running:
pango-querymodules > '/etc/pango/i486-slackware-linux/pango.modules'

(lxappearance:10222): Pango-WARNING **: failed to choose a font, expect ugly output. engine-type='PangoRenderFc', script='common'

(lxappearance:10222): Pango-WARNING **: failed to choose a font, expect ugly output. engine-type='PangoRenderFc', script='latin'

(lxappearance:10222): GLib-CRITICAL **: g_utf8_collate: assertion `str1 != NULL' failed

(lxappearance:10222): GLib-CRITICAL **: g_utf8_collate: assertion `str2 != NULL' failed

2)
me2[/]$ wicd
bash: wicd: command not found
me2[/]$ wicd-gtk
Has notifications support True
Loading...
Connecting to daemon...
Connected.
/home/me2/.gtkrc-2.0:6: error: unexpected identifier `Sans', expected character `='
/usr/share/wicd/gtk/gui.py:166: PangoWarning: No modules found:
No builtin or dynamically loaded modules were found.
PangoFc will not work correctly.
This probably means there was an error in the creation of:
'/etc/pango/i486-slackware-linux/pango.modules'
You should create this file by running:
pango-querymodules > '/etc/pango/i486-slackware-linux/pango.modules'
self.wTree.add_from_file(gladefile)
/usr/share/wicd/gtk/gui.py:166: PangoWarning: failed to choose a font, expect ugly output. engine-type='PangoRenderFc', script='common'
self.wTree.add_from_file(gladefile)
/usr/share/wicd/gtk/gui.py:166: PangoWarning: failed to choose a font, expect ugly output. engine-type='PangoRenderFc', script='latin'
self.wTree.add_from_file(gladefile)
displaytray True

I

This is the error I now get when I try and reinstall pango


# installpkg /var/slapt-get/slackware/l/pango-1.34.1-i486-1.txz
Verifying package pango-1.34.1-i486-1.txz.
Installing package pango-1.34.1-i486-1.txz:
PACKAGE DESCRIPTION:
# pango (library for layout and rendering of text)
#
# Pango is a library for layout and rendering of text, with an emphasis
# on internationalization. Pango can be used anywhere that text layout
# is needed; however, most of the work on Pango was done using the GTK+
# widget toolkit as a test platform. Pango forms the core of text and
# font handling for GTK+-2.
#
/sbin/ldconfig: /usr/lib/libbassmix.so is for unknown machine 40.

Executing install script for pango-1.34.1-i486-1.txz.

(pango-querymodules-32:10913): GLib-GObject-CRITICAL **: gtype.c:2720: You forgot to call g_type_init()

(pango-querymodules-32:10913): GLib-CRITICAL **: g_once_init_leave: assertion `result != 0' failed

(pango-querymodules-32:10913): GLib-GObject-WARNING **: cannot retrieve class for invalid (unclassed) type `<invalid>'

(pango-querymodules-32:10913): GLib-GObject-CRITICAL **: g_enum_get_value: assertion `G_IS_ENUM_CLASS (enum_class)' failed

(pango-querymodules-32:10913): Pango-WARNING **: Engine reported invalid script value 10


AND also when I run " pango-querymodules > '/etc/pango/i486-slackware-linux/pango.modules' "

# pango-querymodules > '/etc/pango/i486-slackware-linux/pango.modules'

(pango-querymodules:10594): GLib-GObject-CRITICAL **: gtype.c:2720: You forgot to call g_type_init()

(pango-querymodules:10594): GLib-CRITICAL **: g_once_init_leave: assertion `result != 0' failed

(pango-querymodules:10594): GLib-GObject-WARNING **: cannot retrieve class for invalid (unclassed) type `<invalid>'

(pango-querymodules:10594): GLib-GObject-CRITICAL **: g_enum_get_value: assertion `G_IS_ENUM_CLASS (enum_class)' failed

(pango-querymodules:10594): Pango-WARNING **: Engine reported invalid script value 10

YIBBIDEE-YIBBIDA' ' I downgraded pango to earlier version & its all come good again as I can see.
Checking it from konsole should of been one of the first things I did to see if it displayed an error, but I forgot.
Not to worry.