The Rules say:
Talking in languages other than English, for any reason other than expression, is also prohibited. This includes the excessive use of 'text speak'.
For example : "y do ppl tlk lyk dis n txt spk? i h8 it dun mk no snse 2 ny1".
The definition of 'expression' is difficult, but the rules cover
all of the forum. I don't know, though, about private journals.
Like others have said, if you want to vent there are other journal options:
livejournal,
blogger and
xanga, for example :)