I speak English natively and German fluently enough that I've been mistaken for a native on the street in Germany, though I'm sure it's much easier to pass for native in Germany where you're getting daily exposure to German and people generally expect the people they meet to be Germans, than it would be here in America where you don't have daily exposure to German, and any Germans you meet will expect you to be a non-native speaker. My vocabulary is also smaller than that of a native speaker, even in the best of conditions.
That's two.
I can read Spanish fairly well, but I can't speak it, and probably about the same for French (though I get much less exposure).
That's two and a half or so.
I've had classroom exposure to Japanese and Russian, and book-exposure to maybe half a dozen or a dozen other languages.
Together, they probably bring me up to two and three quarters.
1) If you speak more than 1 language, do you find yourself having any advantages over the average monoglot?
Yes, even the languages that I have had exposure to but don't really speak have been helpful at times.
2) Do you think western languages will remain the 'international' languages, even with Asian countries (such as China) rising in prominence?
Over the next 50 to 100 years I'd say it's likely. After that, all bets are off.
Keep in mind that their foreign scripts and tonal languages often make it extremely difficult for westerners to attain fluency, while vice versa the skill required for fluency is not quite as much.
Meh... The only foreign script that's going to be any more difficult for Westerners than learning Roman would be for a non-Westerner is the Chinese script, and a writing system is a separate thing from a language (it's possible, for instance, to write Russian in the Roman alphabet, ор инглиш ин сррилик, фор дхат мадр).
Two langauges (Korean and Vietnamese) have already abandoned the Chinese script, Korean for Hangul and Vietnamese for Roman. Of the two major languages that still use the Chinese script, both Chinese and Japanese can be romanized, and Japanese is already partly written in Hiragana. If Japanese dropped the Chinese script and went fully to using Hiragana, learning written Japanese wouldn't be any harder for Westerners than learning written English would be for the Japanese.
As for tones and stuff, it's just a matter of what your brain got used to hearing as you were growing up. I guarantee you that it's as hard for the Chinese to deal with the word "squirrels" (which, depending on what sounds you count as vowels, can be interpreted as a six-consonant cluster with no vowels, though generally the "rrr" sound there will be counted as a vowel) as it is for westerners to deal with Chinese tones. Also, westerners are more sensitive to tone than you realize: we're just used to using it to make distinctions at the sentence level (such as between statements and questions) rather than at the syllable level.
3) What is your opinion of conlangs, such as Esperanto and lojban? Could they ever be useful on an international scale, or do you think they will remain an interest only for enthusiasts or learners?
As far as conlangs, I like conlangs that are meant to be fictional natural languages (like Tolkien's languages), but I've always found stuff like Esperanto and lojban silly.