While not an official explanation from Vince Gilligan or the show, generally speaking the Czech Republic is one of the "meth capitals" of Europe. From a study by the European Monitoring Centre for Drugs and Drug Addiction [note the link is directly to a PDF]:
Historically, the use of methamphetamine has been confned largely to the Czech Republic and Slovakia...
...In the 1970s and later, methamphetamine use was largely restricted to what is now the Czech Republic, though it appeared sporadically in the Swedish amphetamines market, and in recreational settings in some countries (e.g. the German dance scene). From around 2000, methamphetamine spread into the drug scenes in the Czech Republic and Slovakia, where it is now used by a high proportion of these countries’ high-risk drug users...
...scale of methamphetamine production in Europe appears to be limited when compared with the overall global fgures (EMCDDA and Europol, 2013). In 2011, the United Nations Ofce on Drugs and Crime received 350 reports of dismantled methamphetamine production sites from European countries, most of these (328) reported by the Czech Republic. Europol has identifed two main producer regions: one in central Europe, focused around the Czech Republic and neighbouring countries,...
And if you search around you'll find the Czech Republic (for various reasons) is indeed a place that some high level meth dealers (on the level of say Heisenberg or Fring) would be aware of.
Edit: Prostitution in the Czech Republic is also legal. Therefore, Saul would likely include it in a place when making such a reference.
