Electrical – 240 V instead of 230 V on a true RMS multimeter

True RMS measurements is a property the manufacturer will be very proud about to share with you as customer. It is usually printed in bold on the meter itself. True RMS isn't printed on the meter in your example, so it won't be one.

Also I own a more expensive brother of this meter and on that one it says 'True RMS' (in bold print).

Third way to tell is from the specifications. Again the manufacturer will be proud to be able to share True RMS properties and I expect it listed high in the 'key features' and next to the 'ACV' and 'ACA' table headers.

In conclusion: No this is not a True RMS meter.

You must be very careful when measuring voltages on the mains, especially in your country where the voltages are absolutely deadly.

The best way to approach this is to make a resistive divider box. This is a simple resistor divider housed in a safe non-conductive project box. Connect the top and bottom of the resistor divider to a line cord with a correctly polarized plug. Then bring the bottom of the divider and the center tap of the divider out to 5-way binding posts or banana jacks. Also route the Earth Ground lead of the line cord to another banana jack, or 5-way, on the enclosure.

Select a resistor divider ratio so you get an output voltage which is both safe to touch and suitable for your scope's input range. Also, select the resistor values so they have a low enough impedance to not affect your scope's accuracy, but are high enough that you are not burning up too much power in the upper resistor and creating a lot of unnecessary heat.

As you will be multiplying all of your scope readings by the inverse of this ratio, choose a ratio which is easy to manipulate mentally - e.g. 10:1, 15:1, 20:1 - but still provides a safe-to-touch voltage level on the output jacks. ( Not that you will be purpously & routinely touching the output terminals, but accidents and slip-ups do happen. )

Make sure you construct this box in such a way and seal it up so there is NO CHANCE of accidentally touching the Hot Wire. You might also include a pilot light to indicate the box is plugged into the mains. You can't be too careful when messing with the power mains!

Mark the resistor divider's ratio on the outside of the box. Multiply all of your scope readings by this factor to get the actual line voltage.