I don't understand how or why the USD/GBP exchange rate is relevant for a German company? Surely the EUR/GBP rate is more the issue?
My day job is importing to the UK from Europe, and i can tell you that although the price has deteriorated, the price of one euro cost me around 85p 3 or 4 years ago, and when it dropped after that to around 80p I didn't see travian gold prices go down... furthermore the current price is around 88p so not massively different to what it is now, and certainly not 12.5%.
At least if travian were to add some extra benefit for the more expensive gold, or if there were some other improvements to the game and/or game play then this would go some way to justifying the huge increase. Less down time on servers would even be a help. An even better reason would be to say that travian are spending some of the extra money on increasing advertising or some other method of attracting new players to the game.
I'd also suggest that maybe for people who spend a lot on gold, there could be a further saving on price for a larger bundle than the current 1600 gold (which if I'm honest doesn't usually last me very long).
I'm not against prices going up, it happens everywhere all the time, I just think it should be proportionate, fair, and usually should have a better reason than that the fictional currency in game is being affected by an unrelated exchange rate.