Error with multiple spelllang set

[fenuks]

Vim's spelllang allows user to specify multiple languages, separated with commas.
With settings like:

set spelllang=en_gb,pl
let g:grammarous#use_vim_spelllang = 1

:GrammarousCheck results in error:

vim-grammarous: Grammar check failed: java.lang.IllegalArgumentException: 'en-GB,PL' is not a language code known to LanguageTool. Supported language codes are: ast-ES, be-BY, br-FR, ca-ES, ca-ES-valencia, da-D
K, de, de-AT, de-CH, de-DE, de-DE-x-simple-language, el-GR, en, en-AU, en-CA, en-GB, en-NZ, en-US, en-ZA, eo, es, fa, fr, gl-ES, it, ja-JP, km-KH, nl, pl-PL, pt, pt-AO, pt-BR, pt-MZ, pt-PT, ro-RO, ru-RU, sk-SK,
 sl-SI, sv, ta-IN, tl-PH, uk-UA, xx-XX, zh-CN. The list of languages is read from META-INF/org/languagetool/language-module.properties in the Java classpath. See http://wiki.languagetool.org/java-api for details.