Today, I'll be at Hungary at working travel. But unfotunatly only in Debrecen. Too far to Budapest.

Wow! How much distance do you need to travel, from home to Debrecen?

About  400 kilometers. I live in Lviv. Its relatyvely close to Poland and Hungary.


Maybe, I will ask my client about EP.

Are there any German members here? Do you feel the following senteces correct? I mean the last word at the end of each line (eins/welche, keinen, eine/welche):

Wir haben kein Brot. Geh und kauf eins/welche.
Möchtest du Tee? - Nein, ich trinke keinen.
Öffne das Fester! Ich brauche Luft. - Auch ich brauche eine/welche.

This is obviously a test for non-countables' pronouns. Bread, tea and air are (can be, "Brot" can be an exception if you mean a specific loaf of it) non-countables - here "Stoffbezeichnungen" -  in German that go with indefinite pronous.

Wir haben kein Brot. Geh und kauf welches. (you mean the uncountable thing, the concept)
Wir haben kein Brot. Geh und kauf eins. (You mean "one loaf of it". Could be considered colloquial)
Möchtest du Tee? - Nein, ich trinke keinen.
Öffne das Fester! Ich brauche Luft. - Auch ich brauche welche.

Thank you!
So, you can use welcher, welche, welches for non-countables (Stoffbezeichnungen)? I had the feeling that there should be some rule for this but I haven't found it anywhere.

Möchtest du Tee? - Ja, ich möchte welchen - Is it correct too?

Correct, but considered (slightly) colloquial.

You might want to read here on indefinite pronouns.