First pPost

Usually you capitalize everything in a title "First Post", but this is pretty informal, so it doesn't matter.

This is my first post in eEnglish.

"English" is a proper noun, so it gets capitalized.

I usually write in fFrench because for my job, because I need to improve my writting skills in this language.

"French" is a proper noun, so it gets capitalized.
I removed "in this language" so it sounds smoother. Since you already said you "usually write in French", when you say you need to "improve your writing skills", the reader can assume you are talking about writing in French.

However, I wanted to write something in eEnglish because since I passed my B2 eEnglish exam in fFebruary, I have almost ignored this language.

"English" and "February" are proper nouns, so they should be capitalized.

Yes, I watch a lot of Netflix series in eEnglish, but I don't practice my speaking or my writting.

"Writing" has only one 't'
I capitalized "English" again
I took out the "my" after writing, you don't technically need it. Since you already said "my speaking", the "my" can also apply to the "writing"

It's not easy to keep the fluidity in different languages if you don't practice them too mucenough.

This is my first post in eEnglish.

I usually write in fFrench because for my job I need to improve my writting skills in ithis language.

Repeating "in this language" sounded a bit repetitive!

However, I wanted to write something in eEnglish because since I passed my B2 eEnglish exam in fFebruary I have almost ignored this language.

I usually write in fFrench, becauseut for my job, I need to improve my writting skills in this language.

'Writing' not 'Writting'.
Any name of a place, person, month, etc. begins with a capital letter e.g. Italy - Monday - Arabic - May - Sarah.

However, I wanted to write something in eEnglish because since I passed my B2 eEnglish exam in fFebruary, I have almost ignored this language.

Don't forget your commas and capital letters!

Yes, I watch a lot of Netflix series in eEnglish, but I don't practice my speaking or my writting.

It's not easy to keep the fluidity in different languages if you don't practice them too mucenough.

Makes more sense to say 'enough' instead of 'too much'.

This is my first post in eEnglish.

Language names are capitalized in English.

I usually write in fFrench because for my job, I need to improve my writting skills in thisat language.

"In this language" is unclear to me, because it suggests English.

Consonants are only doubled in the -ed and -ing forms if they have a short vowel sound (i.e. it doesn't sound like the letter name). Generally, a VC+e word will have a long vowel, as long as the consonant is not R. Examples:
"Hop": short vowel, becomes "hopping" and "hopped"
"Hope": long vowel, becomes "hoping" and "hoped."

However, I wanted to write something in eEnglish because since I passed my B2 english exam in fFebruary, I have almostmore or less ignored this language.

"B2 English exam" is a little redundant, so we don't need it.
"Almost" is odd with "ignored" here, although it could work with other verbs or with an adverb added. With "almost ignored" by itself, the implication would be that you managed to do some work, but just barely, but that doesn't sound correct in context.

Yes, I watch a lot of Netflix series in eEnglish, but I don't practice my speaking or my writting.

In English, "my" can be part of a parallel structure.

It's not easy to keep the fluiditretain fluency in different languages if you don't practice them too much.

"Keep" by itself sounds odd here, when it's not a question about possessing something.
"Too much" implies that you are practicing them excessively.