We assume you know your trade as a translator, or are willing to read up on it, elsewhere. A few words specifically about translations of RKWard plugins, though:
RKWard plugins were not translatable until version 0.6.3, and were mostly not written with i18n in mind, before then. Thus you are going to encounter rather more ambiguous strings, and other i18n problems than in other mature projects. Please do not just silently work around these, but let us (or the plugin maintainers) know, so we can fix these issues.
Many RKWard plugins refer to highly specialized terms, from data handling and statistics, but also from other fields of science. In many cases, a good translation will require at least basic knowledge of these fields. In some cases, there is no good translation for a technical term, and the best option may be to leave the term untranslated, or to include the English term in parentheses. Do not focus too much on the 100% mark of translated strings, focus on providing a good translation, even if that means skipping some strings (or even skipping some message catalogs as a whole). Other users may be able to fill in any gaps in technical terms.