Please make it possible to add suffixes.

Actually this is to add a "ping" when citing someone, but not necessary to talk directly to that person, this just notifies that person that they are being cited in a mention (it could be a simple quotation from something said or made earlier by that person, or could be an indication given to someonelse to contact the mentioned person):

The suffix would then vary depending if it means "to" ("さんへ" in Japanese), or "about", "from", "by", "for", or if this is just a politeness forula (similar to a "dear") or if it means that this mentioned person is "known", or "recommended", or a "possible" (untested); and it would viable only if the reply in which the mention is made includes that mention in a text in Japanese: in that case the suffix should be part of the response in Japanese, after that mention.

Then if you reply in Japanese and includes "@John" mention, where "John" does not even know Japanese and was cited with his sentence in English, adding "さんへ" will not help John (and readers of your reply may thing that John knows Japanese, when in fact it was just cited).

The only politeness is the (separate) notification that Josn will receive in his own prefered language because he was cited. Others can only know that John was notified. If you expect a reply by John, you would have first to know if he can read your reply, or you would add some text in English just beside the "@John" mention, so that he can read it in the context of your reply, even if the rest of your reply is in Japanese.

So IMHO, this prefix is just a visual indicator and should remain symbolic and generic.

I don't see any better choice than the single symbol "@", which is used by convention, but it could be also an icon or the emoji for a person, possibly tuned by MediaWiki if this is a user name and Mediawiki knows their gender stated publicly by that person in their preference).

If you want to clarify what you mean beside that ("Speaking to X only", "I expect a reply from you X", "thanks to X", "X said to me", "X published...", "X presented...", "X thinks that", "X votes for...", "X rejects...") the text of your reply should clearly state that...

Note that you may need to cite several persons ("@X and @Y approved @Z's proposal"): no "To" meaning implied , so "@Xさんへ and @Yさんへ approved @Zさんへ's proposal" would be very inappropriate ! But the "@" could be an icon or emoji like a "storm strike", or a "male/female/neutral/bot" icon representing the cited user.

Note that you also don't need to use this mention" widget when replying in the same thread to a message posted by that person: in the Mediawiki syntax such replies are just indended, in LiquiThread you just attach a response subthread below the original thread kept as parent. and in that case the link is implicit in that context. Mentions are when a message needs to speak about any other person for any reason and that reason should be exposed clearly.

The "mention" should in fact just generate an appropriate link to that user's profile, that person will be notified (and will reply to your "ping" or not) and others just need to see clearly the generated link that should be usable (so even showing the "@" is not really needed, if the link itself is prominently rendered as identifying a contactable/identifiable person. This maps to what is made in Mediawiki with the conventional Template:@ (alias Template:Ping in many wikis): no symbol, no icon, but a simple way to properly format a link to a user page (possibly hosted on another wiki, or another social site).

"さん" is an honorific title, as is "Mr.".

https://en.wiktionary.org/wiki/%E3%81%95%E3%82%93#Suffix

and "さん"-"へ" means "to Mr./Ms.".

In Japanese, it is rude to refer to another person without adding a suffix.

I understand the "へ" or "to" is a problem, but I feel uncomfortable not using "さん".

Japanese people will not use this feature because they will feel as if they are suddenly being treated rudely on a forum.

Yes but this applies only in a Japanese context, and meaningful only to a user speaking Japanese. And the "ping/@" uis supposed to cite anyone, even those you don't know, you are not speaking to directly, and not Japanese. And is the honorific title appropriate for everyone? What is that "user" is not a person, but a bot engine, or an organization, in fact anything that could just have an account? Or is not an adult. Remember that in many cases you'll cite "somebody" without knowing "who" he is:

• "Hey, @Wikimediaさんへ, when you accept my article?"
• "This page was last edited by @128.12.13.14さんへ."

Do we need to use some properties of the target "user" account to get some knowledge about how to cite that user ? Isn't the unique name of the account sufficient and appropriate for all contexts? For now the only additional property we have is the gender (if the "user" fills that info in their preferences): male, female, neutral (plus "bot" when they've been authorized). All the other info is in the public user page (if it was created by that user) and their edit history (including votes and talks). There's no such info for IP users (that don't have any personal account but temporarily share some IP account that may have been used or may be used later by someone else): preferences are limited to their browsing session (or the visitor's browser cache) but not stored in preferences of the wiki (eventually a part of these preferences will be transmitted for a specific edit, but Wikimedia restricts which parts of this data can be publicly exposed or stored, except the IP and timestamp for edits in histories which are required for legal reasons as well as to help fighting vandalism, but in most cases it is not publicly exposed, notably not for read-only actions).

The best you can do, if you know you're citing a physical person in a message posted in Japanese, is to add the Japanese honorific title appropriately after the "@user" mention, inside your own text. And it should not be considered rude at all if you omit that title for a person that is not Japanese (that will also won't like to have thir user name transformed or "decorated" in a way that that person cannot even read or understand: that "さんへ" may be interpreted very rudely (as if it was hiding some insult, each time you mentioned that person).

As well you may cite a person without even creating any sentence, e.g. as part of a long list of persons, or in tabular data column or bulleted list: this forced honorici peopel would then spread everywhere and would obscure the list. The best alternative is to use a small custom icon or emoji to replace the "@" in order to explicit to readers of your message that you are citing someone. This symbol makes no assumption of rudeness or politeness. It is neutral. The only politeness or honor will not be in the visible message, but in the notification that this user will receive (on the wiki or via some other channels connnected by that user on the wiki, according to user's own preferences).

The "さん" is an honorific title that can be used regardless of gender, and for inanimate objects and organizations. So it is odd to readers that it is not used in the Japanese UI. For example, Twitter and Facebook also use it for the same function in its Japanese UI.

Sometimes I feel that English and Japanese have opposite grammars. The English word "#999" is written with the Japanese word "999番". When we use prefixes in English words, on the other hand, we use suffixes in Japanese words. When translating an English message board into Japanese, that becomes the biggest problem.

If it's a multilingual UI, I'd like to see the option of suffixes available.

Thanks for your comment. So Japanese like to add a magnifying "dear" or "saint" qualifier after every person, organisation, object, or idea ? Is this "さん"/"San" really honorific or just something that is used to explicitly make some precautionary distance between the talker and the absent person/thing that annot reply immediately?

When I look at translations, the "さん" alone becomes "M.", but as a suffix it varies a lot: "Mme", "Mrs", "Sir", or nothing (notably with a pronoun), and in some circumstance the translator needs to add some specific honor like "great", "saint", "grand", (by invention/imagination) and it is often inconsistant across translators. But when I look at postal addresses in Japan, or entries in diaries, it is never used. Same thing in simple lists, or nomintaive citations outside any sentence or given opinion, or in signatures. I don't think it is really honorific, I could just call that a contextual "particle" with no real meaning except to differenciate what someone says for himself from someone/something he's just citing: that's a form of isolation, a "wall" or "guardrail" around the cited person/thing.

Much like if we used quotation marks around the cited person/thing (quotation marks can be marked vocally by a difference of tone/stress or some pause, but here it seems that Japanese uses a specific particle word, in order to avoid any change of tone (which would alter the meaning, as Japanese tone is strongly semantic, even if it is not written when using kanas)

That's really odd...

I know this seems strange to users of languages with less complex honorific systems, but it is simply customary. Some people might walk down the street completely naked, but you would think it rude. It's the same as not addressing someone with "さん".

For more information about "さん", please refer to your Japanese textbook.