fun readLangStreams(baseLang: String, baseStream: InputStream, langStreams: Map<String, InputStream>): Map<String, Map<MsgId, MsgStr>>
Read translations from *.lang files available as InputStreams
baseLang - the base language, from where the translatable strings originate
baseStream - the InputStream for the *.lang file of the base language
langStreams - a Map containing the InputStreams for the other languages
(the keys of this map are the names of the *.lang files without extension)