First, "I started this thread before I had been to Latvia" is completely normal English. Although I often disagree with наша шведская приятельница Hanna on questions of politics, when it comes to English grammar I can promise you that she is very trustworthy!

Marcus -- you're right that, technically speaking, it might be more correct (or at least more clear) to say "I had started this thread before I had been to Latvia." However, it's a mistake to think that the simple-past form "I started" necessarily excludes a past-perfect interpretation. As Seraph says, the past-perfect is often used for emphasis, and also for precision and clarity. But even when the past-perfect is not used, it's still possible (as in this case) to take the simple-past verb as being "logically" past-perfect.

Some alternative wordings:

I (had) started this thread before visiting Latvia.
I (had) started this thread before I went to Latvia.
I (had) started this thread before my trip to Latvia.
I (had) started this thread before going to Latvia.

In each case, the past-perfect "had" is optional -- one can use it or not use it, and it still sounds normal either way. However...

I (had) started this thread before I was in Latvia.

Hmm, this one sounds just a little bit uncolloquial and ESL-ish, although it's not "wrong." Seraph was correct that the verb was is not quite right here, although I can't really explain why. But it's a matter of style, not grammar.