What is a substitute inner tube for 27x2.125" tyres (in Prague)?
Tl;dr: use 700c or 29&quot;
27x2.125&quot; is very rare.  27x2⅛&quot; would be more likely, and wouldn't quite be the same thing, but for tubes the answer would be the same.  Generally though, 27&quot; tyres are narrower than this, up to about 40mm or 1 1/2&quot;
Schwalbe says that for 27&quot; tubes you should use 700c, which are very common.  The bead seat diameter of 700c is 622mm; for most 27&quot; sizes it's 630mm.  As your tyres are on the wide side, you'd want to use 29x2.1&quot; (as 29&quot; is a marketing name for wide 700c)
Tubes are really rather forgiving.  Examples: I'm currently running a 26&quot; tube in one wheel of my 29er hardtail (a pain to fit, but I'd taken the wrong tube with me and had to get home); I've replaced a tube with a puncture only to find that I'd been riding one old bike for a year on a 26&quot; tyre with a 24&quot; tube.
Note that the width is also given as a range, so one tube might be suitable for 1.5-2.4&quot; widths.  Schwalbe's SV19 is a common example tube, and is specified as suitable for ETRTO diameters from 584mm (27.5&quot;) to 635mm (28x1 1/2&quot;/700B) and widths from 40-62mm.  I wouldn't choose it for the very largest diameter and width specified, but it would work.
For info, tyre sizing systems are bizarre - 1/4 is not the same as 0.25 for sizes in inches, and 29&quot;=28&quot;, while 27&quot;&gt;27.5&quot; (and can be bigger than 29&quot;).