I18n

1. Customise you own I18n solution.
One easy way is to overwrite app/helpers/devise_helper.rb, which is mentioned as “A simple way to show error messages for the current devise resource. If you need to customize this method, you can either overwrite it in your application helpers or copy the views to your application.” in the comment.

Alternatively, an effort to homogenize translations for devise is being made here : https://github.com/tigrish/devise-i18n

2. Setting default locale (config/application.rb)

config.i18n.default_locale = :sv

Brazilian Portuguese (devise.pt-BR.yml)
https://gist.github.com/924555 by @mateusg
https://gist.github.com/766771 by @phstc
https://gist.github.com/628507 by @shadow11
https://gist.github.com/1007304 by @lucascaton
https://gist.github.com/1045501 by @danilofavacho
https://gist.github.com/1326333 for Devise 1.4.7 by @daniambrosio

Bulgarian (devise.bg.yml)
https://gist.github.com/2021786 by @bbatsov

Catalan (devise.ca.yml):
https://gist.github.com/1427701 by @codegram and @jrom

Chinese Simplified
devise.zh-CN.yml (for Devise v2.0)
https://gist.github.com/1693549 by @HungYuHei
devise.zh-cn.yml
https://gist.github.com/482530 by @doabit
devise.zh-CN.yml
https://gist.github.com/1008642
devise.zh-CN.yml
https://gist.github.com/1319590 by @acenqiu

Chinese Traditional (devise.zh-TW.yml)
https://gist.github.com/964066 by @l4u
https://gist.github.com/477126 by @gugod

Croatian (devise.hr.yml)
https://gist.github.com/991538 by @babinho

Czech (devise.cs.yml)
https://gist.github.com/889553 by @bver

Danish (devise.da.yml)
https://gist.github.com/638785 by @jgrau
https://gist.github.com/1710884 by @martinjlowm (Devise 2.0.0)

Dutch (devise.nl.yml)
https://gist.github.com/448125 by @wilco

English (default)
https://github.com/plataformatec/devise/blob/master/config/locales/en.yml

Finnish (devise.fi.yml):
https://gist.github.com/786127 by @akonan
https://gist.github.com/1711301 by @jarijokinen (Devise 2.0.0)

French (devise.fr.yml):
https://gist.github.com/1153706 by @bbenezech
https://gist.github.com/379988 by @tadatoshi
https://gist.github.com/871074 by @jujudellago (update for translated-views devise-1.1.2)

German (devise.de.yml)
https://gist.github.com/520859 (Sie) by @chrugail
https://gist.github.com/624775 (Du) by @mrjfk10
https://gist.github.com/960718 (Du auch) by @punkrats
https://gist.github.com/1496869 (Sie) by @fschwahn (devise 2.0)
https://gist.github.com/1820398 (Du) by @ding280 (devise 2.0)

Hungarian (devise.hu.yml)
https://gist.github.com/716483 by @czettnersandor

Hebrew (devise.he.yml)
https://gist.github.com/503804, by @shmichael (devise 1.0.8)

Icelandic (devise.is.yml)
https://gist.github.com/856333, by @sozialhelden

Indonesian (devise.id.yml)
https://gist.github.com/754652, by @yulrizka

Italian (devise.it.yml)
https://gist.github.com/1170644 by @mcanato

Japanese (devise.ja.yml)
https://gist.github.com/606476 by @yhara

Latvian (devise.lv.yml):
https://gist.github.com/1226317 by @mak-it

Lithuanian (devise.lt.yml):
https://gist.github.com/432788 by @pacifists

Malaysian (devise.my.yml):
https://gist.github.com/977606 by @iqbalhasnan

Norwegian Bokmål (devise.nb.yml):
https://gist.github.com/902531 by @bonkowski and @kjbekkelund

Persian (devise.fa.yml)
https://gist.github.com/939048 by @ryco

Polish (devise.pl.yml)
https://gist.github.com/455635 by @sickill
https://gist.github.com/487808 by @grzesiub
https://gist.github.com/891279 by @marioosh (devise 1.2)
https://gist.github.com/1099061 by @rogal111 (devise 1.4.2)
https://gist.github.com/1235911 by @saepia (devise 1.4.6)
https://gist.github.com/1685976 by @kszksz (devise 2.0.0)
https://gist.github.com/1958933 by @Nowaker (devise 2.0.1 + language fixes)

Portugal Portuguese (devise.pt-PT.yml)
https://gist.github.com/579087 by @alexreisner
https://gist.github.com/1002747 by @pama

Russian (devise.ru.yml)
https://gist.github.com/1242140, by @Antiarchitect
[GEM] https://github.com/korobkov/devise-russian, https://rubygems.org/gems/devise-russian by @korobkov
https://gist.github.com/428668, by @sergio-fry

Slovak (devise.sk.yml)
https://gist.github.com/1295185 by @marospixel

Spanish (devise.es.yml)
https://gist.github.com/2125911 by @ianmurrays
https://gist.github.com/1196295 (Tu) (for Devise 1.4.4)
https://gist.github.com/1133929 (Tu) (for Devise 1.4.2) by @willywg
https://gist.github.com/1111315 (Tu) by @scambra
https://gist.github.com/448139 (Tu) by @pacoguzman
https://gist.github.com/463944 (Tu) by @eduludi
https://gist.github.com/1257123 (Usted) by @ChristopherThorpe, forked from 1196295
https://gist.github.com/1302200 – Spanish Mexican for Devise 1.4.8 by Gabriel Medina Rha7
https://gist.github.com/1330093 (Mexico – Usted) by @albertopadilla
https://gist.github.com/1330091 (Mexico – Tú) by @albertopadilla

Swedish (devise.sv.yml)
https://gist.github.com/440569 by @flojon

Turkish (device.tr.yml)
https://gist.github.com/1564962 by @cihad

Vietnamese (device.vi.yml)
https://gist.github.com/1802699 by @stnguyen

Ukrainian (devise.uk.yml)
https://gist.github.com/1502781 by @kolodiazhnyi