I have two questions.
Alas, I don't really good speak English, but I have very good understanding of engineering drawings, experiences and skills which I think successfully enough to work.
This question is likely to be closed as proofreading. But I think something more general (and useful to later readers) can be made from it.
Your sentence has several flaws. I don't want to over-correct it, because then it would no longer authentically represent your voice. However, there are a couple points I would like to discuss:
I don't really good speak English
This could be expressing one of four things:
For 1 and 2, the pattern is:
I don't really [verb]
This pattern can be used only in the negative. It means "I don't [verb]" but using "really" makes it a weaker statement. This communicates a tone of tentativeness, uncertainty, or possibly embarrassment. Compare "I don't play baseball" to "I don't really play baseball", "I don't speak French" to "I don't really speak French"--the former is simply a statement, while the latter is a kind of apology or denial of expertise, even though the speaker can do those things a little.
3 and 4 are less likely to be what you mean. But they would each mean "I feel I speak English somewhat well, but don't want to claim a truly high level of mastery." "Really" here is just an intensifier; it makes a contrast between "good" (as in, good enough) and "really good" (truly exceptional).
In US English, any of these statements would be unremarkable; 2 is more formal than 1 (and 4 is more formal than 3), but 1/3 are still acceptable in a casual context.
In current US English, adjectives come in front of nouns they modify. Adverbs, when they modify verbs, usually come afterward. But you never put a modifier somewhere that it will split a tighter relationship.
Above, 1 uses this pattern:
[subject] [verb] [adjective]-[direct object]
while 2 uses this pattern:
[subject] [verb]-[direct object] [adverb]
The adjective is more closely connected to the noun ("good English") than the verb is to the direct object ("speak English"), so the adjective stays right next to the noun it modifies, and you get "speak good English".
In the second pattern, however, in current usage, the relationship between the verb and its direct object ("speak English") is closer or more tight than the relationship between the verb and its adverb ("speak well"). So we put the adverb after the direct object to get "speak English well". (In older speech, this was not always the case--so we have archaic-sounding "mark well my words" vs. modern "teach them well and let them lead the way")
This is kind of an unusual turn of phrase: something that's just 'good enough but not great' isn't usually described as successful. More importantly, your source phrase:
I think successfully enough to work
has confusing modifier placement. By itself, this phrase sounds like you're saying "I am capable of thinking adequately well." This is because "successfully enough" is placed right after "think", so it seems to be an adverbial phrase modifying "think" (as we discussed above).
You really need to be saying what you do 'successfully enough'. So an appropriate revision might be "I have a very good understanding of engineering drawings, which is enough to make me successful."
From a style perspective, I would probably say "...but I have a very good understanding of engineering diagrams, which means I can communicate effectively."
But it really depends on what you mean by "successfully enough"--what is it that you think you mean you can do well.