The first time I went to US, I was pretty confident in my English but natives didn't understand me. I could repeat the same, grammatically correct sentence 3 times and they still wouldn't get it.
Then I realized that in English (especially American English) you have to accentuate the right syllabus or they won't understand you. Say you're going to "mounTAIN view" instead of "MOUNtain view" and they have no idea what place you're talking about.
That's with every dialect of English. English has phonemic stress, i.e. the way you stress a word can change its meaning or make it unrecognisable unless you're speaking to an English-speaker familiar with people who speak English coloured by whatever your native language is. This is a particular problem for French-speakers, since French doesn't have phonemic stress.
British and Irish people will probably understand French-accented English better than Americans because of greater exposure to French.
I remember a grad student who went through a similar incident. While I appreciated his frustration, I also understand the anxiety of the poor server trying to remember how to mix a Woahtuh before my colleague decided to enunciate the coda.
Then I realized that in English (especially American English) you have to accentuate the right syllabus or they won't understand you. Say you're going to "mounTAIN view" instead of "MOUNtain view" and they have no idea what place you're talking about.