Placeholders in translations

[Tekki]

As mentioned in #124 it would be nice to have placeholders in translations. This would allow us to create correct sentences in languages that don't use the subject-verb-object word order.
The main problem: Who is going to update all the translations after this change?

[sse450]

Just a question. If you implement this, will the old translations work as is if they don't use placeholders?

[Tekki]

Just a question. If you implement this, will the old translations work as is if they don't use placeholders?

They will no longer work at places where the English terms have been changed. To take the example of the other issue:
As soon as Are you sure you want to delete Invoice Number: is replaced with Are you sure you want to delete Invoice %s?, none of the existing translations will work at this place because they don't know this new term. This isn't an acceptable result. The translations (all of them) have to be changed in a way that they display the same text as before. And then those who take care of them can start with linguistic improvements.