Okay, I got it working. File name has to be `filetypes.Kotlin.conf`.
`~/.config/geany/filedefs/filetypes.Kotlin.conf`: ```INI [styling=C]
[keywords] # https://kotlinlang.org/docs/reference/keyword-reference.html primary=abstract assert break case catch class const continue default do else enum extends final finally for goto if implements import instanceof interface native new package private protected public return static strictfp super switch synchronized this throw throws transient try volatile while true false null as fun in object typealias val var when by constructor delegate dynamic field file get init param property receiver set setparam where actual annotation companion const crossinline data expect external infix inline inner internal lateinit noinline open operator out reified sealed suspend tailrec vararg field it # https://kotlinlang.org/docs/reference/basic-types.html secondary=Double Float Long Int Short Byte NaN Void # documentation keywords for javadoc doccomment=author deprecated exception param return see serial serialData serialField since throws todo version typedefs=
[lexer_properties=C]
[settings] lexer_filetype=C tag_parser=C extension=kt mime_type=text/x-kotlin
[build-menu] FT_00_LB=_Compile Kotlin Class FT_00_CM=kotlinc "%f" FT_00_WD=
EX_00_LB=_Execute Kotlin Class EX_00_CM=kotlin "%eKt" EX_00_WD=
EX_01_LB=Execute Kotlin _Script EX_01_CM=kotlinc -script "%f" EX_01_WD= ```