You can override assets for each locale - thanks to Xuzaca (Blizz) for posting the info.
This is very important when you keep in mind that Global Play is coming.
List of Locales
deDE = German
enGB = English (used in the UK | AU | NZ)
enSG = English (used in Singapore)
enUS = English
esES = Castilian Spanish
esMX = Latin American Spanish
frFR = French
itIT = Italian
koKR = Korean
plPL = Polish
ruRU = Russian
zhCN = Simplified Chinese
zhTW = Traditional Chinese
You can overwrite existing Blizzard assets or even your own assets by putting them into the right path. Your Base path will be the default, and will be over-ridden by any other locale that you define. Hence, your Base should theoritically be in English being the most widely-spread langauge, and adding additional locales when needed.
How to do it
Import your base Assets into "Base.SC2Assets\Assets\Textures", then using the list above you can import overrides into their own folders/paths.
Have the same name as the asset they're supposed to override
Are in the correct, locale-specific archive (Base.SC2Assets vs. koKR.SC2Assets)
Have the same path within that archive as the asset they're supposed to override
(Base.SC2Assets\Assets\Textures\x.dds vs. koKR.SC2Assets\Assets\Textures\x.dds)