When a conlang is brand new, there is no choice in the matter– development and tutoring must occur in English (or what ever you and your audience’s language is). To attempt to use your conlang from the onset as the community language is to set the entry requirements so high only the most talented polyglots in the world would be able to join– and they probably wouldn’t want to join when the community doesn’t exist yet.
I got started thinking about then when browsing the demandoj.tk website, which is a Q&A site, but entirely in Esperanto. This makes is an especially high barrier for newbies. Interlingua’s website has the same problem– it is all in Interlingua, you have to read an Interlingua link to find the English page! Now in the defense of auxlangs, their stated goal is to be an L2 of a community with many different L1s. So they have to manage hundreds of L1 communities and move them as fast as possible into the community that uses primarily the L2. It would be unweildy to cater too much to too many L1 communities.
In the world of toki pona, normally we mix English and toki pona. Some people try to use toki pona all the time and I literally, don’t understand them. There are too many concepts that can’t easily be expressed in toki pona. You can try to talk politics and say you don’t like such-and-such a public policy in tp, but you can’t say much intelligent or intelligible after that.
Another option is to explicitly allow diglossia, mixing languages at the phrase level. This was suggested for toki pona with tokipinglish, but in the case of tp, it really breaks the rules of the game.
Recommendations for Community Building
Choose your target L1 communities. For example, if I ever finish writing my own conlang, the target L1 community is English. If speakers of other L1′s happen to find my conlang, then great.
You might decide to target people who speak English, French or Russian (not a bad idea, since conlangs are unusually popular in Europe and there are many talented polyglots there)
If I focus on just English speakers, it allows me to focus on the two communities: learners (who will mostly talk about linguistics, language learning, the merits of conlangs) and competent L2 speakers (who will probably talk about the same things, but will also talk about other things at a high level, entirely in the L2-conlang)
Obviously there isn’t a crisp distinction between the two groups. Competent L2 users will need to participate in the other community to help teach and keep the pipeline of fans moving.
But the sort of websites and social conventions are going to be completely different. For example, the competent L2-conlang-using community will be more likely to have social conventions that are hostile to newbies (i.e. they always use the L2, they don’t simplify,they might use diglossia, but not necessarily in a L1-learner friendly fashion), they might be tired of hearing learner questions, (How do I say my name in Klingon?).
So go forth and build two communities.