Recently, I was a bit surprised to find out that not all Dialogflow system entities are equally supported in all the languages that Dialogflow supports.
For example, if you go to the documentation page, you see the following number entities for English:
Compare the same to French (select the FR tab):
As you can see, there are 6 types of number entities for English, but only two for French. Since @sys.number-integer is used quite commonly in English language agents, you should know that such an agent (that is, one that uses @sys.number-integer) will not work in French.
Why this matters
I have been helping some clients from non-English speaking countries build Dialogflow agents by creating them first in English, and then they will translate it to their native language.
If you follow a similar workflow, you should use entities which are available across all the languages that you are using in the agent. So as you design your agent, you need to make sure the system entity you are interested in is also available in the non-English language.