I started this thread before I had been to Latvia.

[Marcus]

Why is Past Perfect used in the second part of the sentence, not in the first? The Past perfect usually denotes the action which was done before another action, not after.

[Seraph]

Second part of the sentence could use simple past, like 'before I went to' or 'before I saw' or 'before I traveled in', 'before I visited' etc.

The past perfect is often used to emphasize, or to carry some connotation that there is something noteworthy about it.

[E-learner]

This is only a wild guess on my part, but I wonder if there is a grain of truth in it:

In essence, what matters the most for the author is not the time when they were in Latvia, but the time when they were not. First, they hadn't been to Latvia, and after that they started the thread (however silly it may sound).

It's like they want to say, "I started this thread when I had not been to Latvia yet", but for some reason it doesn't sound good enough, so they change "when I had not been to Latvia yet" to 'before I had been to Latvia".

[Marcus]

Can'y we say: "I had started this thread before I was in Latvia"?

[Seraph]

... "I had started this thread before I was in Latvia"?

This looks ok to me. (I might use a different verb than 'was', but I think it is all ok. It's ok when 'was' is referring to a definite time/condition, and that is known.)

[Throbert McGee]

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.

[Throbert McGee]

This is only a wild guess on my part, but I wonder if there is a grain of truth in it:

In essence, what matters the most for the author is not the time when they were in Latvia, but the time when they were not.

Ahhhh, yes, this is also a very good and subtle observation!

Hanna is contrasting her point-of-view NOW with the point-of-view that she had for the years and years and years before her trip to Latvia. Because the time of "not being in Latvia" was one long continuum, that favors a simple-past construction instead of a past-perfect construction.