How to indicate you are going straight? I live in a country where people drives on the right.

Say I am riding my bike like this:

    - main road
    vvvv
    vvvv      - narrow path
    vvvv      vv 
    \    \   |  |
     \    \  |* |  &lt;-- I want to get here
      \    \ |  |
       \    \|P |  &lt;-- pedestrians crossing
        \       |
         \      |
          |     |
          |     |
          |   * |  &lt;-- I am here, going straight
          |   ↑ |
          |     |
           ^^^^^
         main road


That is, I am going straight and want to continue like this in an intersection. For some reasons, people typically turn to the left in the crossroads (it is the main road), but I do want to go straight (a narrower path).

This means that pedestrians typically cross on point (P) without paying much attention to cyclists, despite us having preference. They check the road, though.

If I would go to the left I would indicate it with my arm. However, is there any way to indicate that I am going straight? I think on putting my hand straight but I am not confident that this will be understood by my fellow cyclists or pedestrians.
It's difficult to say without seeing photos of the road layout.  However, if the road itself continues round to the left, you should indicate right when you're leaving it.

In general, you should think about the topology of the road, rather than its geometry.  It doesn't matter that you're following a geometrically straight line; you're still leaving the road for a separate carriageway, and you need to signal this intention.

The default assumption for someone crossing the road is that you'll continue along the carriageway you're already on.

This said, of course, common sense should always prevail!  Never do anything that feels dangerous (for you or for someone else).