Edit `src/main/resources/fess_config.properties`:

Find the `supported.languages` property (around line 202) and ensure your language code is included:

```properties
supported.languages=ar,bg,bn,ca,ckb_IQ,cs,da,de,el,en_IE,en,es,et,eu,fa,fi,fr,gl,gu,he,hi,hr,hu,hy,id,it,ja,ko,lt,lv,mk,ml,nl,no,pa,pl,pt_BR,pt,ro,ru,si,sq,sv,ta,te,th,tl,tr,uk,ur,vi,zh_CN,zh_TW,zh
```

**Optional configurations:**

query.json.default.preference=_query
# Default preference for GSA queries.
query.gsa.default.preference=_query
# Language mapping for queries.
query.language.mapping=\
ar=ar\n\
bg=bg\n\
bn=bn\n\
ca=ca\n\
ckb-iq=ckb-iq\n\
ckb_IQ=ckb-iq\n\
cs=cs\n\
da=da\n\
de=de\n\
el=el\n\
en=en\n\
en-ie=en-ie\n\
en_IE=en-ie\n\
es=es\n\
et=et\n\
eu=eu\n\
fa=fa\n\
fi=fi\n\
fr=fr\n\

    String QUERY_GSA_DEFAULT_PREFERENCE = "query.gsa.default.preference";

    /** The key of the configuration. e.g. ar=ar<br>
     * bg=bg<br>
     * bn=bn<br>
     * ca=ca<br>
     * ckb-iq=ckb-iq<br>
     * ckb_IQ=ckb-iq<br>
     * cs=cs<br>
     * da=da<br>
     * de=de<br>
     * el=el<br>
     * en=en<br>
     * en-ie=en-ie<br>
     * en_IE=en-ie<br>

				"application/x-uc2-compressed",
				"application/x-ustar",
				"application/x-vmdk",
				"application/x-wais-source",
				"application/x-webarchive",
				"application/x-x509-ca-cert",
				"application/x-xfig",
				"application/x-xpinstall",
				"application/x-xmind",
				"application/x-xz",
				"application/x-zoo",
				"application/x400-bp",
				"application/xcap-att+xml",

    <sub-class-of type="application/x-bplist"/>
    <glob pattern="*.webarchive"/>
  </mime-type>

  <mime-type type="application/x-x509-cert">
    <alias type="application/x-x509-ca-cert"/>
    <alias type="application/x-x509-user-cert"/>
    <glob pattern="*.crt"/>
  </mime-type>
  <mime-type type="application/x-x509-cert;format=pem">
    <sub-class-of type="application/x-x509-cert"/>