I think that

> 1. Who can put this tag?

Currently, only advances members and corpus maintainers can tag sentences. I think it's possible to allow owners to tag their sentences with "@needs native check" regardless of they rank.

> 2. In what situations should this tag be put?

The tag should be put every time when the sentence is added by a non-native speaker and there is a doubt about if a sentence is natural and/or grammatical.

> 3. Who can check sentence?

Anyone who is fluent (proficient) in the sentence language can check it and leave an appropriate comment then. Though, the decision of removing the tag should be left for the corpus maintainer who claimed his proficiency in the sentence language, a native speaker preferably.

> 4. What is the difference between @needs native check and @check?

Strictly speaking, I see no big difference between those two tags. But, "@needs native check" may indicate that naturalness of the sentence needs to be validated first of all, while "@check" may refer to the need of validating translation quality, grammar, punctuation, etc.

That's right, correct sentence that don't match the original should be unlinked unless a sentence owner wishes to correct it. The latter is undesired, though, if the sentence has its own translations already.

Usually, a sentence with lesser number is the original with the exception of sentences with numbers up to 329712 approximately that originate from Tanaka Corpus (English - Japanese pairs).

Hi, Ramin!
Welcome to the Tatoeba Project!

I think, it's not a good idea to keep such things here. At least, not as "translation" for original sentences. I hope that metadata mechanism will be implemented eventually and then we'll be able to add transliterations, transcriptions, etc. without poisoning the main database.

As far as I know, the sentences have to be indexed by the search engine before it becomes possible to find them via Tatoeba search tool. I suspect that indexing process just wasn't being ran for some time. Maybe due to performance issues Tatoeba experiencing last time.

Try to open audio links in separate tab/window.

お誕生日おめでとう！＾＾

Добро пожаловать на Татоэбу!
Welcome to Tatoeba!

You've chosen the right place, indeed! Tatoeba is, maybe, not the best site to learn languages but definitely one of the very best to find natural and actual sentences in many languages.
Good luck! :-)

Добро пожаловать на Татоэбу! :-)
Новичкам здесь всегда рады, а что касается нового - по части иностранных языков тут этого хоть отбавляй. :-)

おめでとうございます！その調子で続けてください。＾＾

That would be really nice. The era of new Tatoeba looks promising, indeed. :-)

> Isn't information on each word the task of word dictionaries ?

Essentially, yes. So Japanese indices on Tatoeba provide just the dictionary form of words and the sense number in JMDict/EDICT but not the meanings. For beginners it often hard to determine the dictionary form of a conjugated verb or adjective and, thus, to find its meaning in dictionary. Furigana doesn't help much with this issue, it only solves the Kanji reading and word tokenizing problems.

The main purpose of Japanese indices (as I imagine it for myself) is to provide user with comprehensive information on each word in the sentence, including its reading, dictionary form and a reference to its meaning (sense) in current context. This is truly invaluable thing for learners.

At the moment, it seems that Japanese indices are not maintained at all. The reason is obvious: this job not only requires good command in Japanese but also very time consuming. Additionally, there are no any tools to do it on Tatoeba. Sysko said he's going to implement furigana editing features, though. I think, it could be a good start to implement Japanese indices tools, too.

> It seems that per-language font specifications need not be implemented wholesale, but could be instituted one language at a time.

That's correct. CSS provides enough means to implement per-language styles including custom fonts that will we downloaded on the fly (via '@font-face') and all modern (this is the keyword) browsers seem to provide adequate support for these features.

I've heard that font substitution add-ons exist for both Firefox and Chrome browsers though I've never used them.

> Could you please confirm that if I filter the viewed languages, the download time would be affected only by the viewed languages ?

I haven't investigated this question that close, but, according to some reports (sorry, I haven't an URL at hand), Firefox and Chrome browsers won't download any font defined in CSS via '@font-face' unless they found a reference to it (explicitly via 'style' attribute or implicitly or via 'class' attribute) inside the page HTML code. That is, if you filter all unnecessary languages, the fonts for them won't be downloaded either. On the other hand, according to the same reports, other browsers, IE in particular, seem to download all fonts defined in CSS no matter if they used on the page or not.

> How can the CSS "guess" what language it's processing ?

This is possible via 'lang' attribute. Tatoeba already has information about the sentence language so each sentence may be provided with appropriate 'lang' attribute. There's also 'lang' selector in CSS so it's possible to select and set different styles for different languages.

---- About fonts on Tatoeba ----

From my personal experience, there's still no ultimate solution for multilingual sites that would provide an acceptable level of usability while staying lightweight and effective. That is, any solution we can invent at the moment will be a compromise solution somehow or other. I see several approaches and each of them has its own pros and contras.
1) Remove all 'font-family' specifications from Tatoeba CSS files.
Extremely straightforward solution. The whole site will be displayed using default proportional font specified in browser configuration. This may cause a disruption of Tatoeba visual design depending on particular font selection. To decrease the impact of this it's possible to work out a set of recommendations for Tatoeba users about their browsers customization, font getting and installing processes, etc. We also can count that many users already have their browsers and OSes optimized for different languages though it may seem like catching at a straw.
2) Use only 'serif' and 'sans-serif' in 'font-family' specifications.
This solution is almost the same as the one above but allows to use different typesets and potentially more design style friendly.
3) Use the most “Web safe” fonts, such as Arial / Helvetica for sans-serif and Times New Roman / Times for sans providing fallbacks to sans-serif and serif typesets accordingly.
This solution allows to count on particular font's look and feel but the result will be highly dependent on user OS type and localization since different versions of fonts may have different number of character sets and glyphs supported. But, apparently, for many localizations this solution will degenerate to the one above.
4) Implement 'lang' property support in CSS and provide different font-family for different languages. For the most complex/exotic cases provide optimized fonts via '@font-face' CSS directive.
Maybe the most elegant solution but requires a good deal of work that will include reworking Tatoeba engine, collecting information about best practices in font selection for different languages, investigating the font availability in different OSes and localizations, getting and optimizing free national fonts, etc. Another drawback is that usage of custom fonts may dramatically increase page download time.

I suspect that the truth is somewhere in the middle but hope my considerations will help. :-)