Why is it so difficult to learn the proper accent in multiple languages?

"T" is "T" when written in capitals. "T" is "m" when written in lower case.

Well, not always. I remember seeing a Russian railroad station on which the large outdoor sign with its name was written in script, and the lower-case T within the name (I can't recall the name, unfortunately) was a crossed T whose downstroke went far below the bottom of the other letters. Now that I think of it, I wonder if that's how a script T was written before the Revolution and the changes in orthography that followed it.

It took some searching, but I finally found a Cyrillic T that was written as I described it. You can see it here in an autograph of Anton Chekhov.

And I remembered the "train station." The place-name I saw may not have been on a train station at all; it was Yalta, in huge, glorious script.

I thought it was a cursive or italic T, regardless of upper or lower cas
It just came out as a normal T when I pasted Бaти Били but as m when the TT "quote" > italicised it to be т

I can't find an upper case example, but check this sign for Dastarkhan, a restaurant I've eaten at in Almaty
The T in PECTOPAH is normal but in the cursive font used for Дастархан it comes out as т

The letters in Dastarkhan, n_rb, are in italics. Cyrillic has different forms of printed letters for normal and italic fonts. You can see the differences on this Wikipedia page if you scroll down to letter forms and typography

Edited by NorthAmerican, again and again, to insert a working link.

Try taking a Cyrillic T and using the > or italic+ signs.

Look:
>Дастархан

Yeah, that's what I'm saying. It looks like an m because it's italic, or a "cursive script" type font , not just because it's lower-case

A 'Bulstat' Card (A form of Bulgarian registration - that both Bulgarian and overseas property owners have), is spelt - in capitals.. БУЛСТАТ
In lower case it is.. булсmаm.

For comparison, here is some text from Alexander Pushkin's "Queen of Spades." The scene describes the return of the old countess from a ball and Hermann witnessing, from behind a screen, all the secrets of her boudoir as she collapses into a chair "hardly living," her head nodding from side to side as if it were not voluntary motion but instead the result of some unseen galvanism.

Note that the letter T is shown as T whether capital or lower case: The Queen of Spades.

I should mention that the stresses are marked because the illustration is from a text I used in school.

Edited by NorthAmerican.

Теремок
>Теремок

Стардог!s
>Стардог!s

Two Russian fast food paces in straight text and then Italic.
You can see that Thorn Tree at least keeps an italicised capital T like a T but an italicised lower-case т comes out like an m

Having said that, the actual logo for Stardogs uses the straight version of т but the italicised version of г, so I guess it's a matter of artistic choice.

Because the top bar of the т is longer on the right, it always looks like "Crapdogs" to me at first glance, which is pretty apt

One of the reasons that so many adults are so bad at obtaining a good accent in foreign languages is that so few language teachers know how to teach people how to pronounce foreign languages. The idea that if you keep on listening to a foreign language you will eventually pick up how to pronounce it just doesn't work for most people. Most people do not know how to listen to the sounds, and even fewer know how to translate a sound they hear into the correct mouth movement. Most people aren't even aware of the relationship between mouth movements and sounds.

People need to be told:
- A foreign language (most of the time) has different sounds to your own language. Practically every sound is different, even the ones we told you were the same on day 1 because we wanted to get you going quickly.
- Your ear is educated to hear the sounds of your own language. Your brain has probably learned to count "similar" sounds as the same, and has therefore a concept of a limited number of boxes and put a variety of things into one box, and only things that are so far off as not to be able to be put into any of those boxes will be heard as "different". At the moment, you are probably putting some different foreign sounds into a box, and therefore mishearing it as the same as a sound in your own language. You need to unlearn this, and start hearing some new boxes, so that you can hear (for example) that t in Spanish is not like t in RP English.
- Foreign language sounds require different mouth movements from the sounds in your own language. For most people, much the quickest way to acquire (near) correct sounds in foreign languages is to become aware of what your mouth is doing, and put your mouth to doing the right thing for the sound in that language.
- There is a good interaction between the last two things in this list.

People taught in this way will have a much higher success rate in picking up a good accent. Unfortunately this is so rare, either because so few language teachers know anything about it, or else can't be arsed to do it properly. Few language primers say anything about it either.