I know nothing about transliteration of Latvian names in Russian, but "ds" is pronounced as [ts].
http://ja.forvo.com/word/rihards_dubra/

Sometimes we write names not according to its original sounding but according to tradition. Besides, "s" that most Latvian male names end in is not easy to pronounce when it comes after a consonant. Many of such names was borrowed from other languages and then "latvianized" by adding "-s". We tend to write such names without "-s" though it's possible to find both writings (Рихад and Рихардс, in our case).

Can't you simply write "Рихарц"?

Absolutely not. Though we pronounce "Рихадс" very close to "Рихарц" :-)

Is "Шварцвальд" difficult to pronounce? ☺

Why would it be? o.o

There's no "s" in "Шварцвальд" :-) Anyway, "чёрный лес" is much easier to pronounce. ;-)

ц is [ts], isn't it?

Only in languages that have no "ц". :-) We don't distinguish t and s in this sound, it sounds as a single whole for us. And what is more, we can pronounce "ts" as two sounds. :-)

OK. ☺

For us Japanese, つ is one sound.
We can distinguish between な and んあ. ☺

なな (七, nana) means "seven".
なんあ (南ア, nan'a) means "South Africa".

なな -> нана
なんあ -> нанъа
Perfectly distinguishable. ;-)

ぜにん（是認） approval
ぜんいん（全員） everybody
ぜんにん（善人） a good person

> Are you implying that you can't hear the difference between the above 3 words, and that you personally pronounce all 3 the same way?

They all sound totally different, but I know it's difficult for some non-native speakers to distinguish them.

善人 and 前任 sound the same to me, though. ☺

ц is really [ts] compressed so much that they fuse into one whole. I mean, if you try to pronounce [ts] quicker and quicker, I'm sure you'll eventually get [ц] :)