Add the possibility to switch from other language to english by the -L argument.

[ArkadiuszMichalski]

Description of the Issue

Am I blind and only I can't do it? Argument -L works for all langs (excluding english, custom english and pig), basically only first is important.. I can't switch interface to english from other langs via -L argument. Why I need this:

• because it's faster than go to Prefs >> General >> select....
• because -L argument does not modify nativeLang.xml file, so after the job is done I can close the window without having to change the language back again (repeat above step).
• because with -multiInst I can open next window in other language at the same time (without above step).
• because arguments can be added to the system context menu or used as shortcuts - it's just more convenient and faster.

Steps to Reproduce the Issue

1. Run Notepad++ in other language than english and close it.
2. Try open Notepad++ with -L argument in english.

Unfortunatelly this not work:

notepad++.exe -L
notepad++.exe -Len

But that should mean the default language (english). Or any other value, like -Ldefault, -Len, -Lnot_recognized (so use english).

Debug Information

Notepad++ v7.9 (32-bit)
Build time : Sep 22 2020 - 03:24:22
Path : D:_test\NPPClean\notepad++.exe
Admin mode : ON
Local Conf mode : ON
OS Name : Windows 7 Ultimate (64-bit)
OS Build : 7601.0
Current ANSI codepage : 1250
Plugins : none

[ArkadiuszMichalski]

Refs:
http://www.lingoes.net/en/translator/langcode.htm
https://learn.microsoft.com/en-us/linkedin/shared/references/reference-tables/language-codes

We have two forms of providing these codes:
en-gb or en-GB
or
en_gb or en_GB

Would also be useful if we convert given value to one format: xx-xx (lowercase).