Currently, translate_source_text is a variable that provides the source text being translated. A new variable, translate_target_language, should be created to provide the language that is being translated into.
Filters can be used to detect if a translation is oddly short or long based on the size relative to the source text length. However, this is not ideal when the target language is, for example, Chinese or Korean - the lengths should be quite different, because they are fundamentally different in structure.
In TranslateHooks::onAbuseFilterFilterAction, ::onAbuseFilterComputeVariable, and onAbuseFilterBuilder, register a new variable that is computed using $handle->getCode()