If it is really unusual, or very specific (or you consider that knowing the difference is important), some of us (myself included) use tags to specify that a sentence is only colloquial / spoken / vulgar / formal / something else.

I can't read Finnish at all, so I'm replying to this, courtesy of google translate:

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I have seen some colloquial expressions in a database, so I also added some. (Only the ones I would use when I did not have a dialect expert.) Are there some guidelines that you should know about adding a language? I would imagine that the student of the language would be puzzled to run confusedly into both book and speech expressions, so separating them might be useful.
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1. I agree with what @Aiji said above - tags is one way to distinguish "formal" from "colloquial".
2. It would not be entirely correct to think that language students are the only people who use this website. It can also be used by translators, journalists. Even for spellchecking purposes for your native language. For machine translation as well.
3. A mix of formal, informal, colloquial, vulgar, polite sentences without any tags can be kind of confusing if you learn to speak or communicate from scratch, like a child does, or like a machine that learns a language using some AI algorithms. But if you're learning English from Finnish, for example, you already know Finnish, and reading a Finnish sentence you already know whether it's formal/informal/vulgar. The English sentence should be the same.

If the formality doesn't match (very informal colloquial Finnish sentence is translated by a formal English sentence, or a polite Finnish sentence is translated by a vulgar English sentence), then yes, it's confusing, but, from my point of view, such a translation is wrong and you need to report it to the person who translated it or to your language corpus maintainer. The translation should convey the meaning, the connotation, and the level of formality.

I totally agree with what deniko said.

I think that formality is just one of the many aspects of a language that can be confusing for learners the first time they see it. But once you get it, it’s not a problem any more. Correct me if I’m wrong, but what you said can apply to, say, future tense. It’s confusing for beginners who only know about the present tense to be shown sentences in future tense, so let’s separate sentences by tense (actually, some people are doing this already, using tags like https://tatoeba.org/jpn/tags/sh...ith_tag/6704).

For more information about how to add tags, see https://en.wiki.tatoeba.org/art...w-to-add-tags.

Personally, I wouldn’t make too much assumptions on how my sentences are going to be used and by who. I don’t like the idea of restraining or changing the way I write sentences just because maybe, a non-native speaker will not understand. Quite the contrary, I think Tatoeba is a good place to add colloquial sentences, because there are certainly enough textbooks out there full of formal sentences. Consider the following guidelines, from https://en.wiki.tatoeba.org/art...ow/guidelines:

• We don't want the awkward, unnatural-sounding translations seen in textbooks to help students understand how another language is constructed.
• We want sentences that a native speaker would actually use.
• Remember that others will be using the translation that you make into your own language to study your language.

If you’re still unsure, you can also ask @Silja’s opinion since she’s the corpus maintainer of Finnish.

Kiitos sun lauseistas Tatoebaan! :) Mä olen lisäillyt colloquial-tagejä mun puhekielisiin lauseisiin jo muutaman vuoden ajan. Sitä varten pitää olla advanced contributor, joksi pääsee käsittääkseni aika helposti. Monesti laitan sekä puhekielisen, että kirjakielisen lauseen, niin kuin näytät säkin tehneen. Luulen, että siitä voi olla apua monille.