Linux CLI client 1.0.16 prints UTF-8 regardless of locale settings

I have a computer (running Gentoo Linux) which uses the ISO 8859-15 encoding for all its terminals. When I run the Keybase client (version 1.0.16) on it, I get what looks like damaged UTF-8 output:

~ $ keybase id
ⶠINFO Identifying creideiki
â public key fingerprint: 01B0 F377 A320 9AB7 FCED 8A67 BF88 06F1 8859 0D8F
[...]

If I change my terminal (KDE Konsole 4.14.3-r1) settings to decode UTF-8, without changing locale, it looks right:

~ $ keybase id
▶ INFO Identifying creideiki
✔ public key fingerprint: 01B0 F377 A320 9AB7 FCED 8A67 BF88 06F1 8859 0D8F
[...]

This is despite the fact that my locale settings do not include any UTF-8:

~ $ locale
LANG=
LC_CTYPE=sv_SE
LC_NUMERIC=sv_SE
LC_TIME=sv_SE
LC_COLLATE=POSIX
LC_MONETARY=sv_SE
LC_MESSAGES=POSIX
LC_PAPER=sv_SE
LC_NAME=sv_SE
LC_ADDRESS=sv_SE
LC_TELEPHONE=sv_SE
LC_MEASUREMENT=sv_SE
LC_IDENTIFICATION=sv_SE
LC_ALL=

sv_SE uses ISO 8859-15, and my setting of LC_MESSAGES=POSIX makes me believe all programs should output ASCII.

Start by reproducing keybase id under the reported Gentoo sv_SE and POSIX locale settings, then compare its output with a UTF-8 terminal. Trace the Linux CLI output and locale-handling entry points; done means the command produces correctly encoded output for the active locale without regressing UTF-8 terminals.