Please update to the latest version (5.1.1.3)
Then try Tools/Options/View and UNcheck “Use OpenGL for all rendering (on restart)”
(You have to restart LibreOffice)
Option 1: using an older version (initially I went back to 4.4.5.2, because I still had the install file; you can get other old versions from Index of /libreoffice/old)
Option 2: temporarily install MS Office, install LibreOffice 5.1.1.3, uninstall MS office (this worked for my co-worker today, but IMO is not a real solution)
Some details:
I had the problem with scalc from version 5.1.1.3 64 bit (on Windows 8.1); the Thai consonants were shown (the initial 5.0 version on my co-worker’s computer didn’t display any Thai characters), but the spacing was uneven for most fonts if I had at least one English character in front of the Thai text in the same cell; (Arial Unicode MS had proper spacing all the time for the Thai consonants); but the wrong vowels were displayed with all fonts and they were at the wrong place too.
For testing I switched to Thai input and keyed in “dbf”, which should look like this: “กิด”
But the “b” instead of showing the Thai vowel for a short “i” displayed the long “i” and off to the right by several consonant widths. The last character (a consonant) was properly shown. It looked roughly like this: “กด ี”
I downloaded the “Noto Sans Thai” font as suggested in another thread, but it didn’t help.
I also downloaded the free MS Excel viewer and installed it in the hopes it would have the same effect as the temporary MS Office install on the other computer, but it didn’t. (Additionally installing the MS Word viewer and the MS PowerPoint viewer didn’t help either.)
Please note: I tried the OpenGL settings AFTER I installed all those MS viewers. So I uninstalled all 3 viewers and LibreOffice then had the option checked again! Unchecking it and restarting LibreOffice once more helped - I did not restart the computer.
HTH