This mixture of languages can complicate translation since sometimes I'm not sure which language an odd word comes from. Here are a couple examples from his diary entry for 16 April 1871:
Abraham F. Reimer (1808-1892) diary, Steinbach, Borosenko Colony, South Russia, 1870-1874, Mennonite Heritage Centre Archive, Winnipeg, Manitoba, Kleine Gemeinde collection, vol. 5907, item 3, p.32.
16. F.[reitag] Des margens so 6 grad warm, des Tages so 14 grad war oft dünkel etwas geregent da haben wir das Ertoflen setzen nach zu hause und Egden ganz verichtet auch hat Tövs des Säen und Pflügen verichtet.
And the translation:
16. Fri. Morning 6 degrees [46° F.], day 14 degrees [64° F.]. Often cloudy, rained a little. Set out potatoes at the house and completely finished harrowing. Toews finished plowing and sowing.
'Let's look at the words that I highlighted. First potatoes. Reimer uses the word Ertoflen, which is neither High nor Low German. The standard High German word for potato is Kartoffel, which comes from the Italian word tartufalo and means roughly "land truffle." (A good source for German etymologies is the translation of Friedrich Kluge's etymological dictionary into English that can be found at this link.The standard Low German word is Ieedschock. The first part of the word means "earth" and is related to the Dutch aarde, High German Erde, and English earth. (I don't know what "schock" means - maybe someone can tell me in a comment.) This is a case where the Low German word is more closely related to English and Dutch than to High German.
So Abraham Reimer has combined the Low German Ieed and the High German tofflen and come with Ertoflen. It took me a while and looking through dictionaries to figure what he was setting out.
The second word is harrow. The High German verb "to harrow" is eggen, and the Low German is äajden. According to Kluge's etymological dictionary, this is a case where High German borrowed a word from Low German. Abraham Reimer used the word Egden, which is the way he spelled the Low German word since Low-German speakers pronounce the letter g with an English y sound. Again it took a bit of searching in dictionaries to find the right word and to realize that Reimer's Egden was just a variant spelling of äajden.
The moral is that it can take a bit of searching and imagination to figure out some words, especially if you are not fluent in both High and Low German, as I unfortunately am not.
BTW, my favorite German dictionaries are
Reverso Easy to use, gives suggestions if you don't spell exactly right, easy-to-follow layout of definitions, gives translations of phrases in context.
LEO Not laid out very clearly, but it has more words and definitions, you have to spell the word correctly to find anything, but it does give you a list of suggestions once you type in three letters, which can be really handy.
Koehler-Zacharias Plautdietsch Lexicon By far the most comprehensive dictionary of Low German. Both Low German-English and English-Low German. Spelling is a problem with any Low German dictionary, and he uses something similar to High German spelling, which makes the dictionary easier to use.
Kjenn Jie Noch Plautdietsch is a decent alternative, but it doesn't contain nearly as many words and goes much further toward a phonetic spelling, which can be hard to use.