Many of the properties used for lexemes, senses and forms only make sense between two lexemes for the same language. As far as I can tell, we don't have a way to add a constraint for that yet.
Some examples of properties where it could be used:
- auxiliary verb, e.g. German "sehen" -> "haben"
- collective noun for animals, e.g. English "bee" -> "swarm"
- synonym, e.g. English "chips" -> "crisps" (one sense), "fries" (another sense)
If the property is used on a sense or form and/or the property points to a sense or form, the language of the corresponding lexeme should be used.