The Windows Registry contains an Adobe/InDesign/<version>/User Interface Locale Setting DWORD value which encodes the current application language.

• Under Windows XP systems the complete path of that magic key (ID CS4) is: HKLM/SOFTWARE/Adobe/InDesign/6.0/User Interface Locale Setting

• Under Vista 64-bit [and Windows 7] you'll find the key at: HKLM/SOFTWARE/Wow6432node/Adobe/InDesign/6.0/User Interface Locale Setting (according to Igor Freiberger).

The screen capture below shows how the Registry Editor (Run › regedit) displays the UI locale value from a French Windows XP system:

InDesign User Interface Locale Setting viewed from a French Windows Registry (XP).

Note that the locale code is 4, corresponding to the French UI. Now suppose I want to see what InDesign looks like in Italian. There's nothing to it! From the Registry Editor, let's double-click on the User Interface Locale Setting field and change the value to b (hexadecimal) or 11 (decimal). Here is the display when I relaunch InDesign:

InDesign's Italian UI.

It's a good idea to backup the settings from the Registry Editor before playing with other values:

1) Right-click on the key (6.0) on the left panel

2) Select Export

3) Type a file name (e.g. myIndesignLocale)

4) In the Export Range zone, make sure that Selected branch is checked, then save.

Now, if you want to revert to the initial locale setting, you just need to double-click myIndesignLocale.reg. It's also possible and easy to edit as plain text a reg file for each specific locale. The only data you need to put in the Registration File will look like this:

Windows Registry Editor Version 5.00
"User Interface Locale Setting"=dword:0000000b

The last line contains the hexadecimal 8-digit code of the locale: 0000000b in our example (Italian UI).

Actually I didn't find any documentation about this encoding. I share with you the result of my investigation:

en_US 01 00000001 English (US)
en_GB 02 00000002 English (UK)
de_DE 03 00000003 German
fr_FR 04 00000004 French
ja_JP 05 00000005 Japanese
es_ES 06 00000006 Spanish
pt_BR 07 00000007 Portuguese (Brazil)
sv_SE 08 00000008 Swedish
da_DK 09 00000009 Danish
nl_NL 10 0000000a Dutch (Netherlands)
it_IT 11 0000000b Italian
nb_NO 12 0000000c Norwegian Bokmål
fi_FI 13 0000000d Finnish
el_GR 14 0000000e Modern Greek
cs_CZ 15 0000000f Czech
pl_PL 16 00000010 Polish
hu_HU 18 00000012 Hungarian
ru_RU 19 00000013 Russian
tr_TR 22 00000016 Turkish
ro_RO 23 00000017 Romanian
uk_UA 30 0000001e Ukrainian
he_IL 31 0000001f Hebrew (Israel)
ar_AE 32 00000020 Arabic (UAE)
zh_CN 33 00000021 Chinese (China)
zh_TW 34 00000022 Chinese (Taiwan)
ko_KR 35 00000023 Korean

(Any undefined value will turn the InDesign UI locale to en_US.)