When using a resource pack with a custom font, you need to restart the game for the changes to the font to take effect.

Steps to reproduce:

1. Download, install, & apply a resource pack with a custom font in Global Resources. Here is one such kind of pack: http://mcpedl.com/minecraft-font-pack/
2. If using the previously mentioned pack, or another one that only changes non-Latin characters, then switch to a language that uses characters that are changed by the pack.
3. Notice that the custom font is not applied. (UPDATE: Or only partially applied - see attached screenshots.)
4. Restart the game.
5. Notice that the custom font is applied now.

UPDATE 2017-08-29: This is now partially fixed.

Sometimes, when you apply the pack and load up a world, the font is not applied at all (Figure 1). Other times, it is partially applied (Figure 2). When you restart the game with the pack still applied and load up a world, the font should be fully loaded (Figure 3).