Where do I download Windows Vista language packs?

The recommended method is via Windows Update, per Windows Vista integrated help or the online help here:

http://windowshelp.microsoft.com/Windows/en-US/Help/35a1b021-d96c-49a5-8d8f-5e9d64ab5ecc1033.mspx

Please scroll down for Vista Service Pack 1 MUI’s.
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Click to open Windows Update.

In the left pane, click Check for updates, and then wait while Windows looks for the latest updates for your computer.

If any updates are found, click View available updates.

Select the MUI that you want, and then click Install updates. If you are prompted for an administrator password or confirmation, type the password or provide confirmation.
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However, if you do not have any updates, or if, for me, you have updates but it never displays any MUI’s, you have to look for it.

Keep in mind that installing MUI’s is very slow compared to Windows XP.

Per this link from Ars Technica:
http://episteme.arstechnica.com/eve/forums/a/tpc/f/99609816/m/855006884831

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MUI’s in order -

Chinese (PRC)
Chinese (Hong Kong SAR)
English
Spanish
Swedish
Japanese
Portuguese (Brazil)
Norwegian
Dutch
Finnish
French-
Russian
German
Danish
Korean
Italian

16 32 bit packs:

www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-zh-cn_ac12404fd3350e08ba766c9ab774eb61ecfa7a45.exe
www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-zh-hk_b485acb744566b62d9cde2464f64068882d97d2d.exe
www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-en-us_c38cb05f3146b642c95743633998808fa5229875.exe
www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-es-es_a163c4bbc9c34f2e299c057ef8861f0fa3d8a76d.exe
www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-sv-se_0bb9314bb33c9c0cae897bd34147e14e11daa3b5.exe
www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-ja-jp_f563d5c82a3a156fc764fb78dc25cde8856dc251.exe
www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-pt-br_86694c7aba41372e04da9d45ce40309a3141a9e3.exe
www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-nb-no_751c84a87022d16f8270116727fbcdeb4efc841a.exe
www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-nl-nl_6ed633b7ecd8a8ae26d4196010912ae6e4ca804a.exe
www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-fi-fi_cb28440f6b925a98432acc92e59b51fbccdc5169.exe
www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-fr-fr_c5197c4631ff183b3a315f2187a6f02856bb697c.exe
www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-ru-ru_56e37b9fa209bf0334d8813372636b58744b1d3b.exe
www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-de-de_129e9e10085876706cf81f9949c31a6d9ea0a231.exe
www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-da-dk_f619cf28316a0cf623e1ef9a2e5fab630302a921.exe
www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-ko-kr_3339e4062ccbac56eed8a773024c0ffb847853d0.exe
www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-it-it_4e18e2e79d2c77d1a1efd45eef86a54bf5f117ab.exe

16 64 bit packs:

http://www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-zh-cn_c7a1440a03f226a8ada96d05fc4005672360b7e8.exe
http://www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-zh-hk_d5566545afb858c3187e26d2d146d637aafeb1a7.exe
http://www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-en-us_a26f55943f01c1ce94c60d8d276b71a1279b785a.exe
http://www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-es-es_c0d6ad4877e4070ac6c56d3779f11097b0474b05.exe
http://www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-sv-se_d39e406d29d8b6dd731721a6016964263b649223.exe
http://www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-ja-jp_b0a3ce38426e0a4214a0c65476fe627e2f9017b0.exe
http://www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-pt-br_6196e1a16c11d0790f07c4bdebf6cf493ba389c2.exe
http://www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-nb-no_edbdac0a00118fe2113115b24319b009078e9d7b.exe
http://www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-nl-nl_d70eeabdf121b18b51fabcda75831ca8b7cef3f3.exe
http://www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-fi-fi_5e849137665b2e505f78b397f6db63dd7cc67048.exe
http://www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-fr-fr_19aa2eb3524742dd3617c4a930b435aa7b3e2c37.exe
http://www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-ru-ru_079c1214fe16627a1759530bb3cd63a269e9a352.exe
http://www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-de-de_2ecd8315fcc93731c126cc8de2dc077c3a2f8bd1.exe
http://www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-da-dk_d642f36bd4aeb50bee1bb9db29f67a75500a6f2f.exe
http://www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-ko-kr_86fb0badb0f282a2b2e154dca33bee4f36f4a21f.exe
http://www.download.windowsupdate.com/msdownload/update/v3-19990518/cabpool/lp-it-it_f37ed5378767169fdfea712c7a22aec1c3deb09c.exe
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For Vista Service Pack 1, this is a partial list of a much more comprehensive one originally from:
http://techblissonline.com/vista-sp1-mui-multilingual-user-interface-64bit-x64-language-packs-download-free/

x64 versions:
Chinese (simplified):
http://www.download.windowsupdate.com/msdownload/update/software/updt/2008/04/lp-zh-cn_715b7879c7fa04bc8fef8f7da5d25cd3d4ac33d3.exe
French: http://www.download.windowsupdate.com/msdownload/update/software/updt/2008/04/lp-fr-fr_6f6013466e73fdd53d3b05afc29035c8717a7f72.exe
German: http://www.download.windowsupdate.com/msdownload/update/software/updt/2008/04/lp-de-de_6dcf03abddc251cca859599c87f421c8df61951f.exe
Italian: http://www.download.windowsupdate.com/msdownload/update/software/updt/2008/04/lp-it-it_beeda327848b3b587e02fa7fc820e2db7832c5b8.exe
Japanese: http://www.download.windowsupdate.com/msdownload/update/software/updt/2008/04/lp-it-it_beeda327848b3b587e02fa7fc820e2db7832c5b8.exe
Korean: http://www.download.windowsupdate.com/msdownload/update/software/updt/2008/04/lp-ko-kr_35b007d62bb85d77563c9c4c7f10fb4b62e5d8c1.exe
Portuguese (Brazil): http://www.download.windowsupdate.com/msdownload/update/software/updt/2008/04/lp-pt-br_551c62dd870c460d9037f1688f6ccfe962c9dcd1.exe
Russian: http://www.download.windowsupdate.com/msdownload/update/software/updt/2008/04/lp-ru-ru_3e442c42f8bae5801178bf42ae7d5141d8190db7.exe
Spanish: http://www.download.windowsupdate.com/msdownload/update/software/updt/2008/04/lp-es-es_ab98cfd30ecf2937d9151a51bafd6183eab03e2b.exe

32-bit versions:
Chinese (simplified): http://www.download.windowsupdate.com/msdownload/update/software/updt/2008/04/lp-zh-cn_f59ef0c461e6ab07aa10364fb7476ecdbfac8087.exe
French: http://www.download.windowsupdate.com/msdownload/update/software/updt/2008/04/lp-fr-fr_465046f5bdfc4298fc50eb319091c2b0a3b38972.exe
German: http://www.download.windowsupdate.com/msdownload/update/software/updt/2008/04/lp-de-de_9d639d8497f912f5c1d9f213eb5f842681a5a518.exe
Italian: http://www.download.windowsupdate.com/msdownload/update/software/updt/2008/04/lp-it-it_ab4a758ef378c57a27fc379daea60141f418a2fa.exe
Japanese: http://www.download.windowsupdate.com/msdownload/update/software/updt/2008/04/lp-ja-jp_838e72b9b4c1d046199bb0a16df22dc54de78b78.exe
Korean: http://www.download.windowsupdate.com/msdownload/update/software/updt/2008/04/lp-ko-kr_30d39777bd7a89e04bf82a0872f0715fc0e2829b.exe
Portuguese (Brazil): http://www.download.windowsupdate.com/msdownload/update/software/updt/2008/04/lp-pt-br_9965ec113186503c771429fa967e8aab845b36fc.exe
Russian: http://www.download.windowsupdate.com/msdownload/update/software/updt/2008/04/lp-ru-ru_479d878f3cf33422dd4271be26d18b4a86ffb858.exe
Spanish: http://www.download.windowsupdate.com/msdownload/update/software/updt/2008/04/lp-es-es_90e8fbb6a08844cca3d6298de5bedbaae278e4ff.exe