This is not possible.
For some stupid reason SOE made parts of the localisation server based, so the English client doesn't work with a German server.
Maybe you can overwrite the German VOs with the English ones, but I think the updater will then re-download the German VOs. So it looks like we have to live with the worst local ever in an MMO (I played DAoC and I know of its problems, but that wasn't half as bad as the German local of EQ2.) or quit playing, 'cause SOE will never change this mess!
p.s. About 1/2 of the "Whine-Posts" are because of the localisation. I really like this game, but 'cause of its localisation I can never recommend it to a German, that can't read English fluently. At least 50% of the quests are still in English. A lot of the 50% of quests that are translated have errors like that the name of a mob in the quest does not match the name of the mob to kill. If you know English you have at least a chance the suggest, what the right name should be.
And then are all the forms of Genglish (German - English), where they change the languages even a few times inside one sentence. And then are problems, that the you only see some code instead of the name. It is horrible!
Judge yourself:
Source:
www.olnigg.de
(This particular Errors might be fixed, but there are still similar errors all around.
)