Maarten De Queecker
It's not only a problem of language, it's also a problem of culture. To put it bluntly: Western People (US, EU) don't think in the same way as Eastern people do. This is the main reason why it's seemingly difficult to accurately translate the word "Aiki". Japanese is a language that's very context based (high-context) whereas English and other European languages are a lot more explicit: if we say "a", we mean "a" and not "b" or "c" or "d", depending on which context the word "a" occurs in.
If we, as Westeners, try to interpret O'Sensei's words, we will do so within a Western framework, ie. any interpretation will be subject to cultural bias. We will unconsciously project our values and ideas in our interpretation. The only people who would have been able to correctly interpret O'Sensei's texts are those who were close to him when he wrote those texts, because they lived in the same context as him.
I could say much the same about any text from any culture - but I think that's more or less a cop out.
As Carsten noted, it's not uncommon to for texts from many different cultural contexts to be read and studied, and nobody questions whether or not it's possible to correctly interpret them.
Translation of the word "Aiki" is not all that difficult, although it requires a certain background - but that background is available to anybody, Japanese or not.
Agreement on the translation is, on the other hand, another matter.