Even though it is very important to be precise in the reading of the letters of the Shema', if one was not, one still fulfills one's obligation. Nevertheless, one should not consider this a free pass to read it without paying attention to the rules, but one must make every effort to read it correctly.
While it is correct for all prayers to be read in Leshon HaQodesh (Hebrew, the Holy Tongue), if one reads the Shema' in a language that one understands, one fulfills one's obligation. However, in such a case also, one must be exacting in it's reading in the same way as one must be exacting when reading it in Hebrew.
When reading the Shema', one's ears should hear the words that one is saying. Even if one did not hear oneself read, one will still have fulfilled one's obligation, provided the sound of the words passed through one's lips. On the other hand, if one read the Shema' in one's mind alone, one will not have fulfilled one's obligation of reading the Shema'.
(See Shulhan 'Arukh, Orah Hayyim, 62: 1, 2. Kaf Hahayyim, ibid., Oth 1, 5)
Shma Yisroel, Shema', Shema, Shema' Yisrael, in a foreign language