I. Mispronouncing Hebrew
The minority of Jews who have received sufficient Jewish education to allow proper prayer in Hebrew face a dilemma regarding their less educated co-religionists. Can a Jew who is incapable of properly pronouncing the prayers lead the community services? There are times when tradition, and even law, would demand it, such as on a yahrtzeit or a kohen leading birkas kohanim. If the Jew cannot pray in proper Hebrew, is he even allowed to pray silently in his garbled reading? And let us not forget those with speech impediments who, despite their adequate education, cannot pronounce Hebrew properly.
The Mishnah (Megillah 24b) raises the issue regarding leading prayers and birkas kohanim. Someone from Chaifah and Beis She’an may not lead because they confuse the alef and ayin sounds and the heih and ches. Rashi offers what could be understood as two reasons why they may not lead: 1) By using an ayin sound rather than an alef in birkas kohanim, they change the blessing into a curse. Additionally, 2) their mispronounced prayer is defective.
II. No One Else
Why does Rashi give both reasons? The Divrei Chaim (vol. 2, OC no. 10) suggests that Rashi is referring to someone who can pronounce the Hebrew properly when he tries hard. Such a person would not be allowed to lead the congregation in prayer because the length of the service makes it extremely unlikely that he will be able to maintain his focus throughout. Birkas Kohanim is much shorter and such a person would be allowed to lead, except that the possibility of his accidentally cursing the congregation forces us to prevent him from leading as well.
This fits with one of the views of Tosafos. The Gemara (Megillah 24b) says that R. Chiya mispronounced Hebrew. However, elsewhere the Gemara (Bava Metzi’a 86a) says that R. Yehudah Ha-Nasi appointed R. Chiya to lead prayers on a special fast day. How could R. Chiya lead prayers if he mispronounces Hebrew? Tosafos (Megillah 24b sv. ke-she-attah) suggest that R. Chiya could pronounce Hebrew properly if he concentrated. In such a case, R. Yehudah Ha-Nasi allowed him to lead prayers.
However, Tosafos (Bava Metzi’a 86a sv. achtinhu) take a different approach. They explain that R. Chiya was the only person capable of leading prayers in that unique circumstance of the special fast. If there is no one else available, then even someone who mispronounces Hebrew may lead. Some later authorities (e.g. Pri Chadash, Orach Chaim 53:12) accept this view in practice.
III. His Way
Returning to Rashi, the Turei Even (ad loc.) offers a different explanation for the two reasons. He suggests that a mispronounced prayer is not defective to someone who normally speaks Hebrew in this improper way. That is how he pronounces the words. If this were not the case, then he would always need to find someone to recite prayers for him. However, when such a person leads the prayer services, which amounts to praying on behalf of others, then the mispronounced prayers are defective for the listeners. This does not apply to birkas kohanim, which is why Rashi needed another reason — turning the blessing into a curse — to disqualify someone who cannot pronounce Hebrew properly. The Chelkas Yo’av (Even Ha-Ezer no. 17) ruled this way regarding chalitzah when the man cannot pronounce Hebrew properly.
IV. Used To It / Normal There
The Shulchan Arukh Ha-Rav (128:48) also quotes two reasons for disqualifying such people from leading prayers, although slightly differently from Rashi’s explanations: 1) they distract the public with their improper pronunciations, 2) their prayers are defective. The practical difference between these two views is significant. According to the former, when the public is used to the mispronunciation, such as an established member of the community, there is no problem with him leading services. According to the latter, only in a place where most people pronounce Hebrew that way is it allowed. The Shulchan Arukh Ha-Rav rules strictly, like the second view.
A source for this second approach is a responsum of the Radbaz (Responsa 1:399). He was asked whether Greek Jews could recite birkas kohanim in Egypt, where their pronunciations sounded foreign and incorrect. He replied that while their pronunciation of the blessings was sufficient for Greece, where all the Jews spoke Hebrew that way, they could not do so in Egypt. The Magen Avraham (53:15) rules like the Radbaz and a similar responsum of the Maharit (1:16). The Beis Shmuel (169:28) and Mishnah Berurah (53:37) rule similarly.
However, the first view also has a precedent. The Rambam (Mishneh Torah, Hilkhos Tefillah 8:12) writes that a teacher may appoint a student who mispronounces Hebrew to lead prayers. Why this exception? The Or Samei’ach (ad loc.) explains that the reason for preventing someone who mispronounces the prayers from leading them is respect for the congregation. Therefore, when the congregation is familiar with the person and his speech there is no slight to the congregation and he may lead. A teacher and classmates know the students and therefore the teacher may appoint a student to lead without concern (cf. Taz, Orach Chaim 128:30).
Most Hebrew dialects today — both Ashkenazic and Sephardic — have apparent deficiencies. For their own communities, where most people speak that way, their pronunciations are sufficient. Even in other situations there are ample reasons to allow the alternate pronunciations, as above. However, other cases are more difficult.
Someone with a speech impediment or who lacks the education to properly pronounce Hebrew is still unable to lead services. He may only do so when no one else can. I can see a sympathetic halakhic authority stretching for leniency in defining such situations. For example, R. Yechiel Ya’akov Weinberg (Seridei Eish, Orach Chaim no. 6 — in the new edition) deems a bar mitzvah boy to be the only person capable of leading services at his bar mitzvah. I suspect that a mourner and someone observing a yahrtzeit would also qualify, despite his mispronouncing Hebrew. This would explain the cases I have seen where a mourner who cannot properly pronounce Hebrew leads prayers throughout the year of his mourning. His courage is commendable and the congregation’s leniency is, I suspect, due to the mourner being irreplaceable in that situation.