Yes, I see what you’re saying.
However, I wonder whether you’re putting too much emphasis on the need to translate the customised text into every language, as opposed to just targeting the most common languages of your users?
Let’s game this out. I think it’s easier to talk about this through examples.
Say you have a forum based in Belgium about cycling . 95% of your users speak either dutch (40%), french (30%), german (15%) or english (10% - tourists). The other 5% speak a mixture of other languages. The default language of the forum is dutch.
Now let’s say we just customise the narrative bot text in dutch, the “fallback” feature is relied on, and the dutch customisation is shown to every user of the forum. Everyone would technically see the same text, but 60% of the forum’s users may not understand it, or only partially (moreover, the french and german speakers may feel it’s a ‘flemish’ forum).
Conversely, let’s say you (with the help of a few mods or power users who are native speakers), also translated the customised welcome text into french, german and english and the ‘fallback’ feature wasn’t present. Now 95% of your forum’s users would see the customised text in a language they understand fully (and relate to), and 5% of your users would see different text.
In this kind of scenario, the fallback feature may be useful for that 5% of cases, but it seems the bigger issue is translating the customised text into the handful of most commonly used languages, rather than that a small minority of users will see different text.