Currently, Textastic only supports monospaced fonts and assumes that all characters have the same width. This is not the case for MS Sans Serif which is a proportional font.

Also, unfortunately, Thai is a very complex script with complex Unicode properties (for example, several indivdual unicode characters can be displayed as one) and this is not supported in the current version of Textastic. I'm currently working on Textastic 3.0 and I might support Thai in that version. This largely depends on if I can make this perform fast enough.
I'm a Japanese user and need a better double-byte support.  The current version displays double-byte Japanese characters with the same width as single-byte chars, which seems strange and degrades readability.  Could it be possible to display double-bytes with the double width of the single bytes?

The other features are really great, especially Navigation Wheel.  I hope Apple will incorporate it into iOS!!!

Currently, it looks like Textastic 3.0 will support double-byte characters and also complex scripts like Thai. I'm rewriting the code editor using CoreText.
Thanks for the reply!
I'm looking forward to v. 3.0!