You're free to do whatever you want, but when writing an English-language comic, left-to-right is a more natural reading order than right-to-left.
Because even if you DO tell people it's right to left, and are very clear about it, the text itself is still going to be read left to right. So your readers' eyes will be going from right to left following the actions in the panel, and having to stop when they get to the speechbubbles and start reading the text left-to-right. So their eyes keep zig-zagging, instead of just flowing naturally with the panels.
This is not to say that right-to-left is inherently a bad pattern - just be aware that the images and the text will be working against each other.
ETA: Here, quick illustration via silly scribble example.
Fig 1: A silly discussion about dog-eyebrows, drawn with the panels arranged from right to left.
Fig 2: Reading order of panels and text marked out. The red arrow is the panel reading order - and it flows pretty easily from right to left. No problem there. The problem comes when the reader's eyes lands on the speechbubbles - they then have to start at the top left of each bubble, and zig-zag from left to right as they read the text, and then go back to the red arrow. It becomes a kind of start-and-stop thing.
It CAN still work just fine, but to me personally it has always felt more natural to read English-language comics in a left-to-right flow.