Translation files live in the [WizTree]\locale folder (usually C:\Program Files\WizTree\locale)
Translation files follow this folder structure:
[WizTree]\locale\XX\LC_MESSAGES\default.mo
[WizTree]\locale\XX\LC_MESSAGES\default.po
where XX stands in for the language code.
These are the valid language codes:
aa Afar
ab Abkhazian
ae Avestan
af Afrikaans
ak Akan
am Amharic
an Aragonese
ar Arabic
as Assamese
av Avaric
ay Aymara
az Azerbaijani
ba Bashkir
be Belarusian
bg Bulgarian
bh Bihari
bi Bislama
bm Bambara
bn Bengali
bo Tibetan
br Breton
bs Bosnian
ca Catalan
ce Chechen
ch Chamorro
co Corsican
cr Cree
cs Czech
cv Chuvash
cy Welsh
da Danish
de German
de_AT Austrian German
de_CH Swiss German
dv Divehi
dz Dzongkha
ee Ewe
el Greek
en English
en_AU Australian English
en_CA Canadian English
en_GB British English
en_US American English
eo Esperanto
es Spanish
es_GL Galician
et Estonian
eu Basque
fa Persian
ff Fulah
fi Finnish
fj Fijian
fo Faroese
fr French
fr_BE Walloon
fy Frisian
ga Irish
gd Gaelic
gl Gallegan
gn Guarani
gu Gujarati
gv Manx
ha Hausa
he Hebrew
hi Hindi
ho Hiri Motu
hr Croatian
ht Haitian
hu Hungarian
hy Armenian
hz Herero
ia Interlingua
id Indonesian
ie Interlingue
ig Igbo
ii Sichuan Yi
ik Inupiaq
io Ido
is Icelandic
it Italian
iu Inuktitut
ja Japanese
jv Javanese
ka Georgian
kg Kongo
ki Kikuyu
kj Kuanyama
kk Kazakh
kl Greenlandic
km Khmer
kn Kannada
ko Korean
kr Kanuri
ks Kashmiri
ku Kurdish
kw Cornish
kv Komi
ky Kirghiz
la Latin
lb Luxembourgish
lg Ganda
li Limburgan
ln Lingala
lo Lao
lt Lithuanian
lu Luba-Katanga
lv Latvian
me Montenegrin
mg Malagasy
mh Marshallese
mi Maori
mk Macedonian
ml Malayalam
mn Mongolian
mo Moldavian
mr Marathi
ms Malay
mt Maltese
my Burmese
na Nauru
nb Norwegian Bokmaal
nd Ndebele, North
ne Nepali
ng Ndonga
nl Dutch
nl_BE Flemish
nn Norwegian Nynorsk
no Norwegian
nr Ndebele, South
nv Navajo
ny Chichewa
oc Occitan
oj Ojibwa
om Oromo
or Oriya
os Ossetian
pa Panjabi
pi Pali
pl Polish
ps Pushto
pt Portuguese
pt_BR Brazilian Portuguese
qu Quechua
rm Raeto-Romance
rn Rundi
ro Romanian
ru Russian
rw Kinyarwanda
sa Sanskrit
sc Sardinian
sd Sindhi
se Northern Sami
sg Sango
si Sinhalese
sk Slovak
sl Slovenian
sm Samoan
sn Shona
so Somali
sq Albanian
sr Serbian
ss Swati
st Sotho, Southern
su Sundanese
sv Swedish
sw Swahili
ta Tamil
te Telugu
tg Tajik
th Thai
ti Tigrinya
tk Turkmen
tl Tagalog
tn Tswana
to Tonga
tr Turkish
ts Tsonga
tt Tatar
tw Twi
ty Tahitian
ug Uighur
uk Ukrainian
ur Urdu
uz Uzbek
ve Venda
vi Vietnamese
vo Volapuk
wa Walloon
wo Wolof
xh Xhosa
yi Yiddish
yo Yoruba
za Zhuang
zh Chinese
zu Zulu
zh_CHS Chinese - Simplified
zh_CHT Chinese - Traditional
Make a [WizTree]\locale\XX\LC_MESSAGES folder on your PC (XX being your language code). Then copy the default.po file from the English folder ([WizTree]\locale\en\LC_MESSAGES\default.po) into it.
The "default.po" file is a plain UTF-8 text file holding every text string the app displays.
Open and edit .po files with poedit (get it at https://poedit.net).
Saving in poedit also builds a compiled "default.mo" file, which is the version WizTree actually reads. Make sure you save so this file gets generated.
If Windows security blocks edits to the C:\Program Files directory, copy the files to another folder to edit them there, then move the finished files back into place.
To try out your translation, restart WizTree and choose it from Options->Language. If your new language isn't listed, double check the folder structure above.