Here is parsed vanilla config with 'initSpeed = 900;' defined in MGun class:
http://configs.arma3.ru/154.133741/configfile/CfgWeapons/MGun.html

And as told in BI's CfgWeapons_Config_Reference:
"In A3 this has multiple usages. positive values override magazine initspeed with the fixed number."
https://community.bistudio.com/wiki/CfgWeapons_Config_Reference#initSpeed.3D0

Seems fixed after today's update.

I can confirm.
After latest Arma 3 update - GUI editor wont work!
Spammed .rpt with:
Error in expression <<null>>

Error position: <<null>>
Error Неверное число в выражении

Error loading control config.bin/RscGUIEditor/controls/Tool_0/
Error in expression <<null>>

Error position: <<null>>
Error Неверное число в выражении

Error loading control config.bin/RscGUIEditor/controls/Tool_1/

and so on...

I found the problem.
Check RscGUIEditor class in UI addon config file:

		class Tool_1: Tool_0
		{
			idc = "<null>";
		};

Also:
class RscButton
{
style = 2;
x = 0;
y = 0;
w = "<null>";
h = "<null>";
and another button classes with wrong "string" parameters.

Made some FIX for GUI Editor: ftp://arma3.ru/arma3/addons/@GUIEditorFix.rar
Please note that this addon only fixes RSCGuiEditor classes, not controls parent classes!

Yeah. Now language control is enabled.
Thank you for quick fix :)

Arma 3 has been updated (1,6Mb), stringtable files now seems valid but Arma still in English language even after steam restart and verifying game cache.

Also adding something like
difficultyList[]={"Veteran", "Elite"};
would be great to disable selecting unwanted difficulty levels on the server.

So soldiers cant wear civil clothes due to geneva convention?? LOL! Nice logic guys :)

Delete file named: YOURPROFILE.vars.Arma3AlphaProfile
from My Documents\Arma 3 Alpha and enjoy :)